Expression and phosphorylation of pRB in stimulated human B cells. Freshly isolated human B cells (1.5 × 10⁶cells/mL) were stimulated with a combination of μ (37 μg/mL) and SAC (0.005%) for the indicated times. Total cell lysates were prepared and 30 μg per lane was subjected to Western blot analysis as described in Material and Methods. Hypophosphorylated pRB and hyperphosphorylated pRB (ppRB) were detected with the RB-PMG3-245 monoclonal antibody. The lane indicated as 0 hrs refers to a sample harvested before mitogenic stimulation of the cells. μ, F(ab′)₂ fragment of rabbit antihuman IgM; SAC,Staphylococcus aureus crude cell suspension.