We have determined whether B cells previously activated by anti-Ig (anti-Ig blasts) are responsive to lymphokines that induce isotype switching. Culture of anti-Ig blasts with a mixture of lymphokines, including BSF-1, resulted in marked secretion of IgM and IgG1, but not other IgG isotypes. The IgG1 response of anti-Ig blasts to lymphokines was 13-fold greater than was observed with splenic B cells. B cell blasts induced by 8-mercaptoguanosine or dextran sulfate did not secrete high levels of any IgG isotype in response to lymphokines alone. An mAb against BSF-1 suppressed the IgG1 response of anti-Ig blasts, but not the IgM response to lymphokines. These data suggest that anti-Ig-treated B cells respond to at least one of the effects of BSF-1.

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