Kinetic studies in cultures containing 2 .times. 105 peripheral leukocytes from patients with acute myeloblastic leukemia revealed extensive, radiation-sensitive increases in thymidine [TdR] incorporation without parallel increases in cell number. Modest and variable stimulation of 3HTdR incorporation was seen with the addition of either leukocyte-conditioned medium prepared with phytohemagglutinin (PHA) or PHA alone. Using the method of limiting dilution, stimulation was always observed and ranged from 3- to 20-fold in individual patients. By mixing small numbers of intact cells with larger numbers of irradiated autologous cells, quantitative evidence was obtained for a cellular interaction between irradiated, PHA-stimulated populations and a subpopulation capable of 3HTdR incorporation. Similar evidence for cell-cell interaction was obtained for normal leukocytes.