In vitro culture of hairy cells were successful in 5 patients with hairy cell leukemia: splenic cells were cultured in methyl cellulose plus PHA-leucocyte conditioned stimulation medium for 7 days. Plating efficiency was 0.5% with colony cells having morphological and cytochemical characteristics of hairy cells. A linear correlation between the number of cells plated and the number of colonies produced was found for cell concentrations varying from 5.10(4) to 5.10(5).