We examined the role of protein kinase C alpha (PKC alpha ) in the stimulation of DNA synthesis of Swiss 3T3 cells induced by bombesin, platelet-derived growth factor (PDGF) and phorbol 12-myristate 13-acetate (PMA). We found that cells in which this kinase had been down-regulated showed a partially abrogated mitogenic response to bombesin. The response to PDGF was unaltered; however, the response to PMA was completely suppressed. The mitogenic effect of maximal doses of bombesin and PMA combined was greater than that of either agent alone, suggesting that bombesin does not fully activate the PKC pathway. Accordingly, bombesin-induced PKC alpha translocation from cytosol to membranes was partial, while that observed with PMA was essentially complete. Moreover, exposure to Ro-31-8220, a PKC inhibitor, had significantly greater effects on the response to PMA than on that to bombesin. Our findings point out different roles that PKC alpha may play in diversely activated cells: while, in the case of PMA, stimulation of this kinase may be necessary and sufficient to induce proliferation, it appears to be necessary only for a full response to bombesin, and redundant among the mechanisms triggered by PDGF.