CD40 plays an important role in B cell activation, proliferation, and Ig class switching. The signal transduction pathway mediated by CD40 was studied using monoclonal antibody (mAb) 626.1 to CD40. Burkitt's lymphoma and Epstein-Barr virus-transformed B cell lines and tonsilar B lymphocytes were treated with the anti-CD40 mAb for various lengths of time. The early events triggered by CD40 were examined by monitoring the changes in tyrosine phosphorylation of cellular proteins with anti-phosphotyrosine mAb. Dephosphorylation of specific proteins ranging between 50-110 kD and the appearance of a 28-kD tyrosine phosphorylated protein were seen within 30 s in human B cell lines. The dephosphorylation was reversed and the 28-kD protein was dephosphorylated in cells stimulated for 1 min. In resting B cells, the appearance of the 28-kD phosphoprotein was observed in 30 s after the addition of the anti-CD40 mAb. The tyrosine phosphorylation of this protein persisted. The patterns of protein tyrosine phosphorylation differed from those induced by an anti-immunoglobulin M mAb. The changes in the state of tyrosine phosphorylation induced by the anti-CD40 mAb were obviated by mAb to CD45, a protein tyrosine phosphatase (PTP) or by the addition of sodium orthovanadate, a broad PTP inhibitor. They were also blocked by protein tyrosine kinase (PTK) inhibitors, herbimycin A and genistein, and PKC and protein serine/threonine kinase inhibitors, H7 and HA1004. In addition, the alteration in the tyrosine phosphorylation of PTKs Lyn, Fyn, and Syk was directly demonstrated. Engagement of CD40 for 30 s induced a transient decrease in tyrosine phosphorylation of these PTKs. These results indicate that the early events in CD40 signaling involve the complex interaction between PTP and protein kinases.