Protein kinase C (PKC) comprises a family of isozymes (alpha, betaI, betaII, gamma, delta, epsilon, theta, eta, lambda/iota [mouse/human], and zeta) which are involved in signal transduction from membrane receptors to the nucleus. Activation of PKC by phorbol esters promotes tumor formation, and from that it was concluded that inhibitors of PKC might prevent carcinogenesis or inhibit tumor proliferation. However, the situation is more complicated because the exact function of the different PKC isozymes is not known at present. They have been shown to be involved in synaptic transmissions, the activation of ion fluxes, secretion, cell cycle control, differentiation, proliferation, tumorigenesis, metastasis and apoptosis. Modulators such as bryostatin-1, phospholipid analogues, PKC-activating adriamycin derivatives, CGP41251, UCN-01, and antisense oligonucleotides directed against PKCalpha, have shown antitumor activity in cancer patients. PKC inhibitors are not specific to PKC, but also interact with other signaling molecules, which may contribute to the antitumor effects. Modulators of PKC have also been shown to influence non-MDR1-mediated and MDR1-mediated antitumor drug resistance. This review is focussed on the role of PKC isozymes in human cell proliferation, apoptosis and antitumor drug resistance, and on the use of PKC modulators as antitumor agents.