Marx & Engels Internet Archive: Letters
Marx to Kugelmann Correspondence
Lenin's Introduction to 1907 edition of Letters to Dr. Ludwig Kugelmann.
Marx to Kugelmann March 6, 1868
Marx to Kugelmann in Hanover July 11, 1868
Marx to Kugelmann December 12, 1868
Marx to Kugelmann March 3, 1869
Marx to Kugelmann November 11, 1869
Marx to Kugelmann April 4, 1871
Marx to Kugelmann April 12, 1871
Marx to Kugelmann April 17, 1871