A simple and reproducible system for inducing and measuring proliferation of normal human peripheral blood B lymphocytes was developed and employed as an assay for BCGF activity contained in supernatants of cultured human mononuclear cells. SAC was used to activate human peripheral blood B cells to develop into blasts with low levels of proliferation, and a marked synergistic effect on the proliferation of these activated B cell blasts was demonstrated when various culture-derived supernatants containing exogenous growth factors were added to the system. Substantial BCGF activity was obtained from culture supernatants of co-cultures of pooled allogeneic mononuclear cells from two donors, who were also stimulated with PHA. Kinetic studies demonstrated that maximal BCGF activity was produced in cultures carried for 72 hr or longer. A linear relationship was observed between the logarithm of dilution of added factor and incorporation of 3H-thymidine in the responding SAC-stimulated B cell blasts, suggesting the applicability of this system in screening and quantitating BCGF activity in supernatants of various sources, such as T cell clones, T cell neoplasms, and functional human T-T hybridomas. Furthermore, the system could potentially be adopted to the long-term culture of proliferating normal human B cells.