A latent MAP kinase kinase kinase activity previously detected after chromatography of PC12 cell extracts on Mono Q [(1992) FEBS Lett. 314, 461-465] has been identified as B-Raf by immunoblotting and immunoprecipitation experiments and by its specific activation with B-Raf antibodies. B-Raf is phosphorylated after stimulation of PC12 cells with nerve growth factor or epidermal growth factor, but this is not accompanied by activation of the Mono Q-purified enzyme whether assayed in the absence or presence of GTP-Ras.