Incubation of Swiss 3T3 or L929 cells with tumor necrosis factor (TNF) leads to the rapid stimulation of several cytosolic Ser/Thr kinases active toward myelin basic protein, the S6 peptide (RRLSSLR), the G peptide (SPQPSRRGSESSEE), and Kemptide (LRRASLG). This confirms the hypothesis that kinases other than protein kinases A and C may be involved in the TNF signal transduction. Chromatography on Mono Q resolved multiple kinase peaks with each substrate tested and moreover revealed a TNF-mediated casein kinase-2 activation in both cell lines, measurable with the specific RRREEESEEE peptide or with the G peptide. The TNF-stimulated myelin basic protein kinases-1 and -2 were identified as extracellular signal-regulated kinases-2 and -1, respectively, based on their elution pattern on Mono Q chromatography, their inactivation by protein phosphatase action, their reaction with phosphothreonine and phosphotyrosine antibodies, and by their migration on sodium dodecyl sulfate-polyacrylamide gel electrophoresis as 42- and 44-kDa proteins recognized by anti-extracellular signal-regulated kinase antibodies.