We had previously found that a mutagenized subline of the mouse thymoma EL4 very efficiently stimulates B cells via direct cell-cell contact, thereby inducing the responsiveness of B cells to cytokines. In the present study, we investigated whether this effect could also be mediated by plasma membranes of EL4 (and other) cells. By equilibrium centrifugation of cell homogenates, four cell membrane fractions of different densities were obtained. These were tested for (a) stimulation of B cell proliferation in conjunction with EL4 supernatant as source of cytokines, and (b) enhancement of B cell proliferation at suboptimal concentration of lipopolysaccharide. It turned out that all membrane fractions from a variety of T lineage cells (mutant EL4, parent EL4, BW5147, P198 thymomas, normal T cells) and B lineage cells (BCL1 lymphoma, X63Ag8 cytoplasma, normal B cells) exhibited similar B cell stimulating activity in both assays. Interleukin 1 activity was not detected in the membrane fractions. Heat treatment abolished all activity showing that protein at least was involved. Either protease treatment or extraction with detergent abolished the activity of subcellular fractions rich in intracellular membranes but not that of fractions most enriched in surface membranes. Finally, erythrocyte membranes also displayed B cell-stimulating activity sensitive to protease and detergent extraction. In contrast, and in confirmation of a previous study, liver cell membrane was inhibitory in the B cell proliferation assay with lipopolysaccharide. In conclusion, the effects of cell membranes did not reflect the unique activity of intact mutant EL4 cells. However, with respect to our data it is conceivable that membrane proteins with relatively nonspecific activity and wide distribution among lymphoid cells could play a role in T cell help together with molecules specialized in cell adhesion and cell triggering.