We have developed a method for measuring the activation or suppression of B cells in culture by determination of immunoglobulins released into the extracellular fluid, using a conventional, nonisotopic assay. To avoid the interference of human serum proteins in the assays, cultures were established in medium supplemented with fetal calf serum. This caused some B cell activation, as reflected by the release of small amounts of immunoglobulins (1.5 to 0.7 microgram/ml IgG, 0.8 to 1.4 microgram/ml IgA, 0.6 to 0.7 microgram/ml IgM on day 7), but pokeweed mitogen stimulation resulted in the release of substantially larger amounts of immunoglobulins (8.2 to 4.6 microgram/ml IgG, 3.8 to 2.9 microgram/ml IgA, 8.0 to 4.0 microgram/ml IgM on day 7). Stimulation with Staphylococcus aureus (Cowan I) also resulted in release of immunoglobulins, with somewhat higher levels of IgM, and the release was inhibited by a suppressive substance obtained from Streptococcus intermedius. Time-course curves for immunoglobulin production by cultures stimulated with pokeweed mitogen and S. aureus showed interesting individual variations, but in general the levels began to rise consistently by day 4 or 5 and maximum levels were reached by day 7. These studies show that determination of extracellular immunoglobulins can be used as an index of B cells activation in routine studies.