Levels of protein kinase C (PKC) isoforms in eight human glioblastoma cell lines and two normal human glial cell cultures were determined. Earlier studies identified PKC-alpha and PKC-gamma in these cell lines but PKC-beta was not present. In this study, PKC-epsilon and PKC-zeta are demonstrated immunologically in these cell lines and also in two normal human glial cell cultures. Protein kinase C-delta was not present. When levels of the four isoforms in the tumor cells were compared to levels in the normal cells, no increase was observed in PKC-alpha or PKC-gamma, but PKC-epsilon was elevated three to 30 times in six of the eight tumors, and PKC-zeta was elevated approximately two times in all of the tumors. Incubation of cell line A172 with phorbol ester for 6 hours resulted in a 48-fold maximum increase in the nuclear PKC-epsilon and a sevenfold increase in the plasma membrane fraction with no change in the cytoplasmic fraction. A similar incubation for 4 hours produced a 0.5- to onefold increase of PKC-zeta in cytoplasmic, nuclear, and plasma membrane fractions. Other researchers have shown that overexpression of PKC-epsilon in fibroblasts results in tumorigenesis, and that blocking PKC-zeta function inhibits deoxyribonucleic acid synthesis. These data suggest that alteration in the expression of PKC-epsilon and PKC-zeta could be a factor in the conversion of normal glial cells to glioblastomas.