The potent tumor promoter 12-O-tetradecanyl-phorbol-13-acetate (TPA) has been shown to have profound effects on many mammalian cell types grown in tissue culture. In the human erythroleukemic cell line K562, TPA causes inhibition of cell growth, attachment to plastic dishes and a flat, irregular cell morphology. Accompanying these changes is a reduction in the expression of two cell surface glycoproteins, Gp-105 and glycophorin, which are specific to intermediate and late stages of erythroid cell development (Fukuda 1981). In this study, we found that TPA treatment leads to a reduction in the incorporation of [35S]-methionine into glycophorin which suggests that regulation occurs at the level of de novo biosynthesis. We also examined the effect of TPA on the biosynthesis of other cellular proteins. We found that TPA treatment leads to a de novo induction of two cytoskeletal proteins, vimentin and actin. To further examine the mechanism of action of TPA, we isolated RNA from K562 cells, translated it in vitro, and examined the products by immunoprecipitation. TPA was found to rapidly reduce the level of glycophorin mRNA activity; a 5-fold reduction was noted as early as 1 hour after TPA treatment. Concurrently, the level of vimentin mRNA activity increased. By hybridization of a human actin cDNA probe to K562 RNA, we also found that TPA treatment leads to a rapid induction of actin mRNA. These results suggest that some of the effects of TPA, including possibly tumor promotion, may be due to both positive and negative transcriptional regulation of specific genes.