Normal human B lymphocytes, prepared from peripheral venous blood, have been stimulated with intact anti-IgM (mu chain specific) bound to an insoluble matrix. The activation event, in a subfraction of human B cells, was associated with subsequent receptivity to the mitogenic effects of exogenously added B-cell growth factor. The ability of the cell population to specifically absorb the B-cell growth factor was dependent upon the time of stimulation with the anti-IgM. Continuous replenishment of the growth factor resulted in the ability to maintain long-term growth-factor-dependent human B-cell populations. These cultured B lymphocytes were shown to specifically absorb the B-cell growth factor, suggesting the presence of membrane receptors for it. The cultured B lymphocytes were routinely maintained in logarithmic-phase growth, in the presence of growth factor, with a population doubling time of 36 hr. These cultured B cells have been utilized in a microassay for the assessment of B-cell growth factor activity that is accurate, sensitive, and precise.