Production of biologically active FL by MSCV-FL leukemic cells. Serially transplanted leukemic spleen cells were transferred to liquid culture and maintained in the presence of KL, IL-3, and IL-7 for 2 weeks before analysis. Conditioned medium (CM) was collected from cells at a density of 10⁶/mL and tested at 1:2 dilution for capacity to stimulate the proliferation of the FL-responsive cell line WWF7 in the presence of 100 ng/mL IL-7 (as described in Materials and Methods). MSCV-FL, CM from GP+E-86/MSCV-FL producer cells; BALB/c SPL, CM from control BALB/c spleen cells; FL, recombinant human FL. Approximate undiluted FL equivalent concentrations: SPL196 CM, 0.8 ng/mL; SPL199 CM, 0.6 ng/mL; SPL203 CM, 0.2 ng/mL; and SPL 204 CM, 7.8 ng/mL.