The effects of the tumor-promoting phorbol ester 12-0-tetradecanoylphorbol-13-acetate (TPA) on the morphology and growth properties of C3H/10T1/2 clone 8 cells were examined. The morphology of these cells was changed within 30 min following treatment with 0.1 micrograms of TPA per ml; they become smaller and refractile with long beady processes. Such changes were observed in both logarithmic and confluent cultures and lasted about 72 hr. Subsequent treatments were much less effective in inducing these changes. Scanning electron microscopy showed cell retraction and rounding as the most significant immediate effects of TPA treatment; many cells remained partially rounded 48 hr afterwards. Long-term surface modifications ascribable to TPA treatment were not detected. TPA had only minor effects on the growth of cultured C3H/10T1/2 cells in the presence of 10% fetal calf serum. Slight increases in plating efficiencies and saturation densities were generally observed in the presence of TPS but not with the related non-tumor-promoting compound phorbol. The cells grew slowly in 1% fetal calf serum and demonstrated serum batch-dependent alterations in their growth properties when exposed to TPA. Under conditions that produce doubling times of 70 hr or greater, TPA, but not phorbol, reduced the doubling time to about 50 hr. Saturation densities were also increased by TPA in 1% fetal calf serum. The effects of TPA on the growth of an oncogenically transformed variant of C3H/10T1/2 were quite different. While minimal effects of TPA were observed when transformed cells were treated in the presence of 10% fetal calf serum, TPA treatment in 1% fetal calf serum significantly inhibited cell growth.