| Marx to Friedrich Adolph Sorge. 4 April |
| Engels to Philipp Pauli. 25 April |
| Marx to Pyotr Lavrov. 18 May |
| Engels to Marx. 24 May |
| Marx to Engels. 25 May |
| Engels to Marx. 28 May |
| Marx to Friedrich Adolph Sorge. 14 June |
| Marx to Wilhelm Liebknecht. 14 June |
| Marx to Pyotr Lavrov. 14 June |
| Marx to Pyotr Lavrov. 15 June |
| Engels to Pyotr Lavrov. 16 June |
| Engels to Pyotr Lavrov. 30 June |
| Engels to Marx. 25 July |
| Marx to Engels. 26 July |
| Engels to Philipp Pauli. 11 August |
| Engels to Pyotr Lavrov. 15 August |
| Marx to Engels. 19 August |
| Engels to Jenny Marx. 20 August |
| Engels to Marx. 25 August |
| Engels to Ida Pauli. 27 August |
| Marx to Max Oppenheim. 30 August |
| Marx to Jenny Longuet. End August |
| Marx to Max Oppenheim. 1 September |
| Marx to Max Oppenheim. 6 September |
| Marx to Max Oppenheim. 9 September |
| Marx to Ida Pauli. 10 September |
| Marx to Max Oppenheim. 12 September |
| Engels to Pyotr Lavrov. 15 September |
| Marx to Ferdinand Fleckles. 21 September |
| Marx to Wilhelm Bracke. 23 September |
| Marx to Wilhelm Bracke. 30 September |
| Marx to Pyotr Lavrov. 7 October |
| Marx to Wilhelm Liebknecht. 7 October |
| Marx to Leo Frankel. 13 October |
| Engels to Ernst Dronke. 15 October |
| Marx to Thomas Allsop. 16 October |
| Engels to Emil Blank. 16 October |
| Engels to Thomas Allsop. 17 October |
| Engels to Ernst Dronke. 20 October |
| Engels to Ludwig Kugelmann. 20 October |
| Marx to Pyotr Lavrov. 21 October |
| Engels to Ernst Dronke. 1 November |
| Marx to Wilhelm Bracke. 6 November |
| Marx to Collet Dobson Collet. 10 November |
| Engels to Ernst Dronke. 13 November |
| Marx to Wilhelm Bracke. 20 November |
| Engels to Johann Philipp Becker. 20 November |
| Engels to Gustav Rasch. End November |
| Marx to Collet Dobson Collet. 9 December |
| Marx to Engels. 11 December |
| Engels to Philipp Pauli. 16 December |
| Engels to Hermann Engels. 18 December |
| Engels to Johann Philipp Becker. 21 December |