In order to define an in vitro culture system allowing growth of single human germinal center B cells (GC-B), we have studied the proliferation and differentiation of human tonsillar GC-B, and subsets thereof, when cultured together with murine EL-4 thymoma cells in the "EL-4 system." The cells were analyzed and compared to resting tonsillar B cells with respect to phenotypic changes, proliferation, Ig secretion, intracellular Ig levels, and growth abilities under limiting dilution conditions. It was found that GC-B differentiated terminally to Ig-secreting cells with the phenotypic features of plasma cells in a similar manner to tonsillar resting B cells. The GC-B proliferated for 4-5 days, followed by a loss of GC-B phenotype and an increase in intracellular immunoglobulin levels. Over a 10-day culture period a larger proportion of the Ig produced by GC-B was IgG and IgA, as compared to resting B cells, indicating that these cells switched isotype more easily or had already switched in the germinal center prior to the culture period. Analysis of frequencies of Ig-producing cells revealed that 1/3.8 of GC-B and less than 1/10 of the centroblast B cell subpopulation (CB-B) differentiated toward Ig-producing cells when cultured in the EL-4 system whereas 1/1.25 and 1/1.5 of peripheral blood B cells (PBL-B) and resting tonsillar B cells did so, respectively. Taken together, these findings show that tonsillar GC-B differentiate in a similar manner to resting B cells when cultured in the EL-4 system, and we conclude that these conditions allow manipulation of GC-B in single cell cultures in vitro.