Engagement of membrane IgM on a number of human and murine B-cell lines induced activation of a Mn(2+)-preferring serine/threonine kinase that phosphorylated microtubule-associated protein-2 (MAP-2) in vitro. B-cell MAP-2 kinase (MAP-2K) activity could be fractionated into two peaks by sequential DEAE and hydrophobic chromatography. Although peak I included two tyrosine phosphoproteins of molecular mass 36 and 38 kDa, peak II showed a single 42-kDa tyrosine phosphoprotein (pp42). Since all kinase activity could be removed from peak II material over an antiphosphotyrosine immune affinity column, it suggests that pp42 is identical with lymphoid MAP-2K. Although peak I activity showed a similarity to peak II with regard to its preference for Mn2+, sensitivity to phosphatase exposure, and resistance to a range of common serine kinase inhibitors, it is not clear whether these activities are related. MAP-2 kinase activity could also be induced by treatment with the phorbol ester, phorbol myristate 13-acetate, suggesting that protein kinase C may also be involved with MAP-2K regulation. Although MAP-2K activity reached a peak response within minutes of receptor ligation, there were differences in the rates of dephosphorylation of pp42 and decline of MAP-2K activity in different B-cell lines. The tyrosine phosphatase inhibitor, vanadate, transformed a rapidly reversible MAP-2K response in BAL 17.2 cells into a sustained state of activation that resembled the kinetics of activation in WEHI-231 cells. The latter finding implies involvement of a tyrosine phosphatase, which opposes the effect of an inducing tyrosine kinase.