Kinetic studies in cultures containing 2 X 10(5) peripheral leukocytes from patients with acute myelobastic leukemia revealed extensive, radiation-sensitive increases in thymidine 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 phytohemagglutin (PHA) or PHA alone. However, 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 capable of 3HTdR incorporation. Similar evidence for cell-cell interaction was obtained for normal leukocytes.