An enzyme-linked immunoassay was developed to quantitate the production of soluble CD23 from cycling B lymphocytes. This molecule has identity both with B cell-derived B cell growth factor and with an IgE-binding factor. B lymphocytes, which had been stimulated for 3 days with phorbol dibutyrate and calcium ionophore, washed and recultured, failed to produce detectable levels of CD23 over the following 3 days. Soluble CD23 was found, however, in the supernatant of cultures where recombinant interleukin 4 had been included. The level of CD23 rose dramatically when anti-Bp50 had also been added. By contrast, anti-Bp50, alone or together with low molecular weight T cell-derived B cell growth factor, failed to promote the release of CD23 in detectable amounts. There was a strong correlation between the appearance of soluble CD23 in culture supernatants and the expression of CD23 on the surface of restimulated cells. The level of CD23 release appeared to relate more to the continued cycling of cells than to their differentiation to immunoglobulin secretion. These findings are discussed with particular emphasis on the role of CD23 as an important multi-functional lymphokine in B lymphocyte physiology.