In AIDS elevated serum Ig levels and autoimmune phenomena indicate that B cells are also involved. The human immunodeficiency virus (HIV) can be cultivated in B cells, and HIV can stimulate B cells. In order to characterize the B-cell dysfunction and conditions for modulating it, functional studies with highly purified B cells were done in four patients with PGL and HIV-positive sera. Data were compared with those from patients with AIDS and normal controls. The experiments consisted of an in vitro study of the differentiation response (IgM/G secretion into culture supernatants) to a T cell-independent polyclonal B-cell activator (PBA) [Klebsiella pneumoniae, KlebsM]. A weak increase in IgM/G levels under stimulatory conditions was characteristic. Addition of recombinant interleukin 2 (rIL2) failed to increase the spontaneous Ig levels. However, coculture experiments using KlebsM and rIL2 resulted in Ig levels like those known from healthy individuals. Patients with frank AIDS did not respond with increased IgG secretion. This indicates that the abnormal B-cell differentiation response to PBAs can be modulated by rIL2 in patients with PGL and partly in AIDS (only IgM).