Verbascoside [1] isolated from Lantana camara is an inhibitor of protein kinase C (PKC) from the rat brain. Half-maximal inhibition of the kinase occurs at 25 microM. Verbascoside interacted with the catalytic domain of PKC and was a competitive inhibitor with respect to ATP (Ki = 22 microM) and a non-competitive inhibitor with respect to the phosphate acceptor (histone IIIS). This effect was further evidenced by the fact that verbascoside inhibited native PKC and its catalytic fragment identically and did not affect [3H]-phorbol-12,13-dibutyrate binding to PKC. The antitumor activity of verbascoside measured in vitro might be due at least in part to inhibition of PKC.