Acquired immunodeficiency syndrome (AIDS) in childhood is characterized by recurrent bacterial infections, a feature common in antibody deficiency disorders. The present study was aimed at investigating B lymphocyte function in 15 children aged 6 months to 6 years with AIDS or AIDS-related complex (ARC). Spontaneous secretion of immunoglobulins by freshly isolated peripheral blood B cells and the generation of immunoglobulin and antibody-secreting cells in lymphocyte cultures after polyclonal and antigenic stimulation were quantified in hemolytic plaque assays. Despite excessive spontaneous immunoglobulin secretion, responses elicited by B cells after in vitro stimulation were depressed in these children. Responses to T-dependent as well as to T-independent stimuli were affected. Studies of immunoregulatory T cells and intrinsic B cell function suggested that deficient precursor B cells and abnormal immunoregulation contributed to the defects in B cell differentiation. These findings indicate that B lymphocyte dysfunction is an integral feature of HTLV III infection in children who clinically present as either AIDS or AIDS-related complex.