The addition to culture dishes of 10-50 ng/ml of the essential diterpene ester of Sapium sebiferum, 12-O-hexadecanoylphorbol-13-acetate (HPA), increased colony formation of normal peripheral blood lymphocytes co-cultured with gamma-irradiated HTLV-I-producing HUT 102 cells. The cells in the stimulated colonies showed an approximately 3-fold increase in the expression of interleukin-2 (IL-2) receptors and a 1.5- to 2.0-fold increase in human T-lymphotropic virus type- I (HTLV-I) p19-positive cells. This biological potency was analogous to that induced by the most potent tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and stronger than that of 12-O-hexadecanoyl-16-hydroxyphorbol-13-acetate (HHPA) isolated from Aleurites fordii.