We studied the signal induced by the anti-TAPA-1 antibody and compared it to the signal induced by anti-IgM antibodies in a human B cell line, OCl-LY8. We found that exposure of these cells to either antibody resulted in a rapid increase in protein tyrosine phosphorylation which was prevented by inhibitors of tyrosine kinases. Tyrosine phosphorylation was an early event in the cascase leading to the antiproliferative effect of the anti-TAPA-1 antibody. However, 2-ME, a reducing agent that is not an inhibitor of tyrosine kinases, prevented both tyrosine phosphorylation and the antiproliferative effect of the antibody. Cells grown in low concentrations of 2-ME did not exhibit an increase in tyrosine phosphorylation in response to the anti-TAPA-1 antibody and were insensitive to the antiproliferative effect of the antibody. In contrast, the same cells maintained in 2-ME were able to induce tyrosine phosphorylation in response to anti-IgM. The use of 2-ME resulted in an increase in intracellular thiols, mostly glutathione. Moreover, compounds that block glutathione synthesis rendered cells susceptible to the antibody, even in the presence of 2-ME. These experiments demonstrate that tyrosine kinases are involved in propagating the antiproliferative signal initiated by the anti-TAPA-1 antibody and suggest that this signal is dependent upon the level of intracellular thiols.