The tumour promoter PMA has been shown to both prevent and induce differentiation of a variety of cell types in culture. The reason for its paradoxical effects is not known. However, it is clear that PMA alters the cell membrane and therefore it is possible that PMA may only be effective in instances where differentiation is accomanied by changes to the cell membrane e. g. myoblast fusion during myogenesis. In this study, its effects on myoblast fusion as well as the appearance of the muscle specific isoenzyme of creatine phosphokinase (M-CPK) which is not fusion dependent is examined. It is shown that M-CPK accumulates in myogenic cultures exposed to PMA although fusion is prevented. PMA is also tested in foetal rat hepatocytes which differentiate and acquire the enzyme tyrosine aminotransferase during culture. There is no evidence which suggests that this change is membrane dependent. The tumour promoter does not prevent the accumulation of tyrosine aminotransferase in cultured foetal rat hepatocytes.