Protein kinase C (PKC), an enzyme involved in signal transduction, responds to diacyl glycerol and also to phorbol ester, a ligand analogous to diacyl glycerol. We have studied the expression of the major isoforms (alpha, beta I, beta II, and gamma) in eight human glioblastoma cell lines. In all eight lines, PKC-alpha mRNA and protein were expressed. In none of the eight did a probe for PKC-beta I and -beta II mRNA give positive results nor were Western blots for PKC-beta II positive. The half-life for PKC alpha mRNA was approximately 16 h and levels of the mRNA were increased slightly following addition of phorbol myristate acetate (PMA) or transforming growth factor-beta (TGF beta). PKC-gamma was present in most of the glioblastomas. In cell line A172, 82% of the PKC-alpha was present in the cytosol with the remainder evenly divided between plasma membrane and nucleus. Thirty minutes after addition of PMA, 33% of the total original protein was in the plasma membrane and 48% in the nuclear fraction. By 21 h, no PKC-alpha was recovered from any fraction. PKC-gamma was also down-regulated in the presence of PMA, but there was no evidence for translocation to the plasma membrane or nuclear fraction. In a more detailed study, translocation of PKC-alpha in the presence of PMA was complete by 10 min, and a major decrease in the PKC translocated to the plasma-membrane fraction occurred some time between 2 and 4 h after PMA addition, while a major decrease in the translocated nuclear fraction occurred some time after 6 h. cAMP alone had no effect on the PKC alpha protein level or distribution, nor did it alter the translocation and down-regulation due to PMA exposure. In these studies the level of PKC-alpha mRNA in tumors was similar to that in normal glial cells.