AbstractThe effect of the phorbol ester tumor promoter 12‐O‐tetradecanoylphorbol 13‐acetate (TPA) on cell invasion was studied using an in vitro assay for cell invasion through a reconstituted basement membrane matrix (Matrigel). TPA inhibited the invasiveness of malignant human fibrosarcoma HT1080 cells. In contrast, WI‐38 lung fibroblasts, which show a very low invasive capacity, were stimulated (3‐fold) to invade Matrigel after exposure to TPA for 48 hours. The inhibitory or stimulatory effects of TPA on cell invasion were correlated with a decrease or an increase in cell motility and collagenase IV activity, respectively. Synthetic diacylglycerols partially mimicked the inhibitory action of TPA on HT1080 cells but failed to stimulate WI‐38 cell invasion. Immunoblots demonstrated that in both cell lines the α and β isoforms of protein kinase C were equally down‐regulated after a 5 hour exposure to TPA despite the basal low level of protein kinase C polypeptide in the malignant cells. Thus, whereas in WI‐38 cells induction of an invasive behavior could be observed in the absence of protein kinase C, in the malignant cells disappearance of the kinase was associated with a non