Peripheral leukocytes from patients with acute (AL) or chronic (CL) leukemia were cultured in vitro, and their behaviour when stimulated by phytohaemagglutinin (PHA) or staphyloccal filtrate (SF) was studied. The response of the cells from AL patients was much higher than the response obtained with cells from CL patients. However, only 2 patients from the former group reached almost normal values when stimulated by SF, while generally the response to PHA and SF were depressed in both groups. Leukocytes from normal subjects showed maximum response at 3 and 5 days after incubation for PHA and SF stimulation respectively. The cells from leukemic patients reached maximum degree of stimulation after a varying delay of a few days. After treatment when remission was induced, a gradual return to normal of the magnitude and kinetics of response was observed in peripheral blood of some patients.