Eduard Bernstein
In 1880 he was accused, together with other activists, in a judicial process with socialists in Kraków. In 1882-1884 he organized socialist groups in Poznań and was eventually imprisoned by Prussian authorities. Being a skilled organizer and publicist, Mendelson personally befriended many foremost European socialists - Friedrich Engels, Karl Liebknecht, Eduard Bernstein, Karl Kautsky, Paul Lafargue, Georgi Plekhanov and others.