The distribution of B lymphocytes expressing surface IgG subclasses has been defined by using immunochemically purified, subclass-specific, F(ab')2 fragments. Studies in normal adults showed 1.16% of peripheral lymphocytes to be positive for IgG. Percentages of IgG subclasses were IgG2, 0.50%; IgG4, 0.40%; IgG1, 0.33%; and IgG3, 0.14%. Dual label experiments indicated that minor subpopulations of normal lymphocytes may express more than one IgG subclass, both on surface IgG as well as in the cytoplasm. Cord blood showed significantly less IgG4-positive cells than were present in adults. Preliminary studies of patients with common variable immunodeficiency showed shifts in cell populations paralleling those previously noted in serum. Some patients with multiple myeloma were noted to exhibit striking shifts in the distribution of B lymphocytes toward a population of cells showing a similar IgG subclass to that of the M-component.