The effect of cyclosporine (CsA) on the CH12 murine B cell lymphoma was investigated to determine whether sensitivity to this agent is retained by malignant B cells. This tumour produces an antibody to bromelain-treated red blood cells and may represent transformation of a B cell with certain activation properties associated with early resting B cells. In in vitro cultures, the growth and proliferation of CH12 were inhibited by CsA in concentrations of 0.1-1.0 microgram/ml; these levels were ineffective against non-lymphoid tumours, although some non-specific cell toxicity was noted at higher levels. IgM antibody production, as measured by enzyme-linked immunosorbent assay (ELISA), was inhibited over the same range. CH12 cells stimulated by lipopolysaccharide, however, were less sensitive to CsA than untreated cells. These studies indicate that malignant B cells may be sensitive to CsA, perhaps reflecting their derivation from a functionally distinct B cell population with enhanced drug sensitivity.