Nerve growth factor (NGF) stimulation of PC12 cells activated two myelin basic protein (MBP) kinase activities greater than 10-fold within 5 min, which were resolved by chromatography on Mono Q. Each enzyme phosphorylated MBP on threonine and was inactivated by incubation with either CD45, a protein tyrosine phosphatase, or protein phosphatase 2A (PP2A), a serine/threonine phosphatase. The effects of CD45 and PP2A were prevented by vanadate and okadaic acid, respectively. Activation of the MBP-kinases provides a mechanism for communication between NGF and intracellular protein tyrosine phosphorylation.