Modulation of protein phosphorylation activities by insulin was investigated in glioma and normal glial cells. Insulin suppressed the in vitro protein phosphorylation of glioma cells in a dose-dependent manner while it stimulated that of meningiomas, neurilemmomas and glial cells. Although gliomas and glial cells contained different species of tyrosyl phosphoproteins before treatment, they expressed similar kinds of tyrosyl phosphoproteins in response to insulin. Insulin increased the activities of casein kinase II and total protein kinase C (PKC) in glioma and normal glial cells. The membrane-bound PKC activity in U373-MG cells was elevated by insulin. The PKC isozymes, including subtypes alpha, beta, delta, epsilon and gamma, were detected in gliomas, but few were found in glial cells. Insulin down regulated the cytosolic PKC-gamma and the membrane-bound PKC-epsilon proteins in gliomas. These results indicate that an altered insulin signaling pathway exists in human gliomas, which might involve differential regulation of PKC isozymes.