| Marx to Josef Valentin Weber. 15 January |
| Marx to Engels. 25 February |
| Marx to Johann Philipp Becker. 26 February |
| Engels to Marx. 28 February |
| Marx to Engels. 3 March |
| Engels to Marx. 5 March |
| Marx to Engels. 6 March |
| Engels to Marx. About 8 March |
| Marx to Engels. 15 March |
| Marx to Engels. 28 April |
| Marx to Ferdinand Lassalle. 28 April |
| Engels to Marx. 5 May |
| Marx to Engels. 6 May |
| Engels to Marx. 12 May |
| Engels to Marx. About 18 May |
| Marx to Engels. 19 May |
| Engels to Marx. 23 May |
| Marx to Engels. 27 May |
| Engels to Marx. 29 May |
| Engels to Marx. 4 June |
| Engels to Carl Siebel. 4 June |
| Marx to Engels. About 6 June |
| Marx to Ferdinand Lassalle. 16 June |
| Marx to Engels. 18 June |
| Engels to Marx. About 3 July |
| Marx to Engels. 5 July |
| Marx to Engels. 1I July |
| Marx to Engels. 21 July |
| Engels to Ferdinand Lassalle. 23 July |
| Engels to Marx. 30 July |
| Marx to Engels. 30 July |
| Engels to Marx. 31 July |
| Engels to Marx. 1 August |
| Marx to Engels. 2 August |
| Marx to Engels. 7 August |
| Engels to Marx. 8 August |
| Marx to Engels. 9 August |
| Engels to Marx. Before 12
Aug. |
| Engels to Marx. 13 August |
| Marx to Ferdinand Lassalle. 13 August |
| Marx to Ferdinand Lassalle. 14 August |
| Marx to Engels. 14 August |
| Marx to Wilhelm Schwarz. 19 August |
| Marx to Ferdinand Lassalle. 20 August |
| Marx to Engels. 20 August |
| Engels to Marx. 21 August |
| Engels to Ferdinand Lassalle. 21 August |
| Engels to Marx. Early Sept. |
| Engels to Marx. 9 September |
| Marx to Engels. 10 September |
| Marx to Wilhelm Wolff. 4 October |
| Engels to Marx. 16 October |
| Marx to Engels. 29 October |
| Marx to Engels. 4 November |
| Engels to Marx. 5 November |
| Marx to Ferdinand Lassalle. 7 November |
| Marx to Engels. 9 November |
| Marx to Engels. 14 November |
| Engels to Marx. 15 November |
| Marx to Engels. 17 November |
| Marx to Engels. 20 November |
| Marx to Ferdinand Freiligrath. 15 December |
| Marx to Engels. 24 December |
| Engels to Marx. 26 December |
| Marx to Ludwig Kugelmann. 28 December |
| Engels to Marx. 30 December |