The biochemistry of human B lymphocyte differentiation to plasma cells is incompletely understood. L-glutamine appears to be required for both lymphoblastic transformation and plasma cell formation in pokeweed-mitogen-stimulated human peripheral blood mononuclear cell cultures. Cells cultured with pokeweed mitogen in glutamine-deficient RPMI-1640 with 10% heat-inactivated and dialyzed fetal bovine serum were unable to incorporate 3H-thymidine or undergo morphologic lymphoblastic transformation assessed at 72 hours. However, 3H-thymidine incorporation could be maximally restored with as little as 0.08 mM L-glutamine or by using nondialyzed heat-inactivated fetal bovine serum, containing approximately. 1 mM L-glutamine. In subsequent cultures, using glutamine-deficient RPMI-1640 with 10% nondialyzed heat-inactivated fetal bovine serum, lymphoblastic transformation was equivalent with or without additional L-glutamine supplementation. However, only cultures with 2 mM L-glutamine supplementation underwent plasma cell differentiation as assessed by cytoplasmic staining with fluorescein-conjugated anti-immunoglobulin. When the kinetics of cellular immunoglobulin synthesis and secretion were analyzed by 3H- leucine incorporation into immunoglobulin, synthesis was 2-5 fold greater, and secretion 3-10-fold greater in cell cultures with 2 mM L-glutamine supplementation. By electron microscopy, only the glutamine-supplemented cells showed development of rough endoplasmic reticulum consistent with active immunoglobulin production. L-glutamine supplementation had no apparent effect on cell recovery, viability, % B cells, % T cells, % monocytes, or % helper and suppressor T cells. Thus, L-glutamine is essential for both lymphoblastic transformation and plasma cell differentiation. Future investigation of the selective nutritional requirements of cultured cells should yield further insights into the biochemical control of immune cell differentiation and function.