While cross‐linking of the membrane IgM (mIgM) molecules expressed on WEHI 231 lymphoma cells induces these cells to undergo apoptosis, we have previously observed that ligation of the mIgD expressed on IgD‐transfected WEHI 231 (Wδ) cells is not associated with induction of cell death. Thus mIgM⁺IgD⁺ Wδ cells provide a valuable reagent for delineating the molecular events which modulate the physiologic outcome of B cell antigen receptor (BCR) engagement. In view of recent data implicating the cytosolic phosphotyrosine phosphatase PTP1C in the regulation of BCR signaling capacity, we used Wδ cells to investigate the potential role for PTP1C in modulating the cell response to BCR activation. The results of this analysis revealed PTP1C to undergo rapid tyrosine phosphorylation following mIgM or mIgD cross‐linking and to associate with a number of other phosphoproteins in stimulated Wδ cells. Among these latter phosphoproteins, one prominent species of about 44 kDa (pp44) which co‐precipitated with PTP1C in mIgM‐ligated cells was not detected in PTP1C immunoprecipitates from mIgD‐ligated cells. The association of PTP1C with this 44‐kDa phosphoprotein following mIgM cross‐linking was also observed in two additional B cell lines representing an immature state of differentiation, but was not detected after BCR engagement in two representative mature B cell lines or in splenic B cells. Initial data concerning the identity of pp44 indicate that this molecule does not represent the Shc, MAPK or Ig‐β proteins and may, therefore, constitute a previously unidentified signaling effector. While the structural and biochemical properties of pp44 require further definition, the findings suggest that BCR‐triggered interactions of PTP1C with pp44 occur only in the context of an immature state of cellular differentiation and the induction of apoptosis. These data therefore suggest that PTP1C interactions with pp44 may be relevant to the transduction of BCR signals which evoke cell death.