The expression of phenotypic markers on B lymphocytes in patients with the acquired immune deficiency syndrome (AIDS), in human immunodeficiency virus (HIV) seropositive individuals, and in healthy seronegative donors was examined by two-color flow cytometry. Patients with AIDS and HIV-seropositive individuals showed an elevated percentage of B cells bearing an activation marker, the transferrin receptor, when compared with donors not infected with HIV. A decrease in the percentage of resting (Leu-8 positive) B cells was also seen in AIDS patients and HIV-seropositive individuals. An increased percentage of circulating, immature (CALLA-positive, CD10) B cells was seen in AIDS patients. These phenotypic changes were accompanied by an increased level of spontaneous IgG and IgM secretion, and increased cell size within the total B cell population and in some B cell subpopulations, in patients with AIDS and in HIV-seropositive people. These results demonstrate that phenotypic changes indicative of in vivo B cell activation and immaturity accompany the polyclonal production of Ig seen in HIV-infected individuals.