Phorbol ester tumor promoters such as 12-O-tetradecanoylphorbol-13-acetate (TPA) alter the morphology and differentiated states of numerous cell types in vitro. It is likely that these molecules are teratogenic for vertebrate embryos in which cytodifferentiation, cell-tissue interactions, and cellular motility are prominent. By examining embryonic effects, I thought that insights into mechanisms might be gained that would not be apparent in adult tissues. The purpose of this study was to characterize the responses of frog (Bombina orientalis) embryos and embryonic cells to TPA. Effects of 10 to 100 ng TPA per ml on whole embryos included epithelial dissociation, inhibition of muscle segment histogenesis and adhesive organ differentiation, and acceleration of cell sheet movements during gastrulation. Possibly correlated with accelerated gastrulation, TPA also induced circumferential rotations of lobopodial blebs in blastula cells cultured on plastic. This capability is normally not seen until later developmental stages and is most prominent in areas undergoing morphogenetic movements. For blastula cells cultured on agar, TPA (1.0 to 10 ng/ml) inhibited cytokinesis but not karyokinesis, leading to the formation of abnormal multipolar spindles. A similar uncoupling of cytokinesis and karyokinesis in adult tissues could lead to mitotic aneuploidy, possibly an important step in tumor promotion. The amphibian embryo should be a useful organism for further studies on phorbol ester tumor promoters.