Medium conditioned by lectin (and/or antigen)-stimulated human peripheral blood lymphocytes has been shown to contain factors, termed interleukin 1 (IL-1) and interleukin 2 (IL-2)1,2, which augment and maintain human T-cell proliferation3–6, respectively. Interleukin 2 (T-cell growth factor, TCGF) requires the presence of a macrophage, or its monokine product, IL-1, for its production by T cells7–9. It has been shown that multiple signals are required for stimulating both T- and B-cell proliferation8–10 but the factors necessary for optimal B-cell mitogenesis have not been rigorously defined. As both B- and T-cell stimulatory agents are present in lectin-stimulated peripheral blood lymphocyte-conditioned media, we studied the effects of the interleukin factors on human B cells. Human peripheral blood B lymphocytes and pokeweed mitogen (PWM)-stimulated B-cell blasts were evaluated for their proliferative response to soluble growth factors, which were obtained from either lipopolysaccharide or phytohaemagglutinin (PHA)-stimulated peripheral blood mononuclear cells. We show here that factors from lipopolysaccharide-stimulated conditioned medium were unable to stimulate initial B-cell proliferation. However, factors from PHA-stimulated conditioned medium stimulated 3H-thymidine (TdR) incorporation in both B cells and lectin-activated T cells. This stimulation could be maintained for multiple re-exposures to the growth factors, which suggests that a soluble factor present in medium conditioned by lectin-stimulated peripheral blood mononuclear cells may modulate B-cell proliferation in a manner analogous to T-cell proliferation.