Hapoel Hatzair
Along with Eliezer Kaplan Sprinzak headed "Hapoel Hatzair" ("The Young Worker") a Zionist socialist faction formed in 1905 and one of the organisations that consolidated to form Mapai in 1930. Its members were pro-British and supported Chaim Weizmann. He was a founder of the Histadrut in 1920 and acted as secretary general of the organisation from 1945 to 1949.