To investigate the role of protein kinase C (PKC) in the growth of astrocytic brain tumors, human glioblastoma cell line U-87 was stably transfected with the antisense complementary deoxyribonucleic acid encoding PKC alpha. The effect of selectively down-regulating the alpha isoform on other PKC isoforms, as well as serum-dependent proliferation and in vivo tumorigenicity, was determined. U-87 cells expressed high levels of PKC alpha and lesser amounts of the gamma, epsilon, and zeta isoforms, and a similar PKC isoform pattern was observed in two other human glioblastoma cell lines. Expression of the antisense PKC alpha complementary deoxyribonucleic acid resulted in no detectable PKC alpha by immunoblotting and a 95% reduction in total Ca2+/phospholipid-dependent PKC activity. U-87 cells expressing antisense PKC alpha exhibited an increase in doubling time in vitro, less serum-dependent growth, and reduced sensitivity to a selective PKC inhibitor, Ro 31-8220. The transplantation of U-87 cells expressing antisense PKC alpha into nude mice resulted in no tumor formation. These observations suggest that the inhibition of PKC alpha may be an important chemotherapeutic target for arresting the growth of glioblastomas.