| Marx to Natalie Liebknecht. 13 January |
| Marx to Hermann Jung. 18 January |
| Marx to Julian Harney. 21
January |
| Marx to Sigfrid Meyer. 21 January |
| Marx to Friedrich Adolph Sorge. 21 January |
| Marx to Johann Jacoby. 4 February |
| Marx to Ludwig Kugelmann. 4
February |
| Marx to Paul Lafargue. 4 February |
| Marx to Pyotr Lavrov. 27 February |
| Marx to Natalie Liebknecht. 2 March |
| Engels to Karl Klein and Friedrich
Moll. 10 March |
| Engels to Rudolf Engels. 10 March |
| Marx to Paul Lafargue. 23 March |
| Marx to Peter Imandt. 30 March |
| Engels to Philippe Coenen. 5 April |
| Engels to Wilhelm Liebknecht. 5 April |
| Marx to Wilhelm Liebknecht. 6 April |
| Marx to Wilhelm Liebknecht. c. 10 April |
| Marx to Ludwig Kugelmann. 12 April |
| Marx to Wilhelm Liebknecht. 13 April |
| Engels to Wilhelm Liebknecht. 13 April |
| Marx to Ludwig Kugelmann. 17 April |
| Engels to Wilhelm Liebknecht. 20 April |
| Engels to Francisco Mora. c. 20 April |
| Marx to Friedrich Adolph Sorge. c. 20 April |
| Marx to Leo Frankel. c. 26 April |
| Engels to Ludwig Kugelmann. 28 April |
| Marx to Wilhelm Liebknecht. 4 May |
| Engels to Wilhelm Liebknecht. 4 May |
| Marx to Wilhelm Liebknecht. 5 May |
| Engels to Hermann Jung. 10 May |
| Marx to Leo Frankel and Louis Eugene Varlin. 13 May |
| Marx to Edward Spencer Beesly. 12 June |
| Marx to Nikolai Danielson. 13 June |
| Marx to His Daughters Jenny, Laura and Eleanor. 13 June |
| Engels to Elisabeth Engels. 16 June |
| Marx to Ludwig Kugelmann. 18 June |
| Marx to an Unknown Addressee. 19 June |
| Engels to Wilhelm Liebknecht. 22 June |
| Engels to Carlo Cafiero. 1-3 July |
| Engels to Wilhelm Liebknecht. 10 July |
| Marx to Leon Bigot. 11 July |
| Marx to Pyotr Lavrov. 12 July |
| Marx to A Rutson. 12 July |
| Engels to Carlo Cafiero. 16 July |
| Marx to Eugen Oswald. 21 July |
| Marx to Nikolai Danielson. 22 July |
| Marx to Eugen Oswald. 24 July |
| Marx to Charles Caron. 26 July |
| Marx to Ludwig Kugelmann. 27 July |
| Engels to Wilhelm Liebknecht. c. 27 July |
| Marx to Nikolai Utin. 27 July |
| Engels to Carlo Cafiero. 28 July |
| Marx to Adolphe Hubert. 28 July |
| Engels to the Mother Superior of the Convent of the Sisters of Providence in Hampstead. Early August |
| Engels to Philippe Coenen. 4 August |
| Engels to Pyotr Lavrov. 9 August |
| Marx to Theodor Koll. 10 August |
| Marx to Adolphe Hubert. 10 August |
| Marx to Hermann lung. 14 August |
| Marx to Adolphe Hubert. c. 14 August |
| Engels to Wilhelm Liebknecht. Mid-August |
| Engels to Marx. 18 August |
| Marx to Engels. 19 August |
| Marx to Engels. 21 August |
| Engels to Marx. 23 August |
| Marx to Engels. 24 August |
| Marx to Friedrich Bolte. 25 August |
| Marx to Jenny Marx. 25 August |
| Marx to Hermann Meyer. c. 25 August |
| Marx to Moncure Daniel Conway. 29 August |
| Engels to Pyotr Lavrov. 3 September |
| Marx to Friedrich Adolph Sorge. 5 September |
| Marx to Collet Dobson Collet. 6 September |
| Marx to Engels. 8 September |
| Engels to Miller & Richard. c. 9 September |
| Engels to Wilhelm Liebknecht. 11 September |
| Marx to Friedrich Adolph Sorge. 12 September |
| Engels to Pyotr Lavrov. 12 September |
| Marx to Hermann Jung. 13 September |
| Marx to Adolphe Hubert. 15 September |
| Marx to Jenny Marx. 23 September |
| Marx to Gustav Kwasniewski. 29 September |
| Engels to Pyotr Lavrov. 5 October |
| Marx to Hermann Jung. 11 October |
| Marx to Hermann Jung. 13 October |
| Marx to John Hales. 14 October |
| Marx to Edward Spencer Beesly. 19 October |
| Marx to Hermann Jung. 19 October |
| Marx to Hermann Jung. 20 October |
| Engels to Elisabeth Engels. 21 October |
| Marx to Edouard Vaillant. 22 October |
| Marx to Hermann Jung. c. 25 October |
| Marx to Hermann Jung. End of October |
| Engels to Wilhelm Liebknecht. 4 November |
| Marx to Hermann Jung. 4 November |
| Marx to Friedrich Adolph Sorge. 6 November |
| Marx to Ferdinand Jozewicz. 6 November |
| Marx to Nikolai Danielson. 9 November |
| Marx to Friedrich Adolph Sorge. 9 November |
| Marx to Ludwig Kugelmann. 9 November |
| Marx to Carl Speyer. 10 November |
| Engels to Theodor Cuno. 13 November |
| Marx to Hermann Jung. 16 November |
| Marx to Wilhelm Liebknecht. 17 November |
| Marx to Julius Johannard. 18 November |
| Marx to Hermann Jung. 18 November |
| Marx to Adolphe Hubert. 22 November |
| Marx to Friedrich Bolte. 23 November |
| Engels to Carmelo Palladino. 23 November |
| Marx to Cesar De Paepe. 24 November |
| Marx to Laura and Paul Lafargue. 24 November |
| Engels to Paul Lafargue. 25 November |
| Marx to Juliusz Balaszewicz-Potocki. 25 November |
| Marx to Juliusz Balaszewicz-Potocki. 29 November |
| Marx to Friedrich Adolph Sorge. 29 November |
| Engels to Pyotr Lavrov. 29 November |
| Engels to Paul Lafargue. 9 December |
| Engels to Wilhelm Liebknecht. 15 December |
| Marx to Laura Lafargue. 18 December |
| Engels to Paul Lafargue. 30 December |