We investigated the immune function of a patient with anergy and acquired hypogammaglobulinemia. Despite normal numbers of B cells and T4+ inducer and T5+ suppressor T cells, this patient's lymphocytes did not produce immunoglobulin, proliferate in response to soluble antigens, or generate helper factors in vitro. In addition, her T4+ T cells did not express la molecules after stimulation by soluble antigen. In mixing experiments, her T cells did not induce immunoglobulin secretion by B cells from a normal, HLA-D-identical sibling; this failure was not due to excessive suppression, since the patient's T cells did not abrogate immunoglobulin production by the normal sibling's T and B cells. Moreover, the patient's B cells secreted immunoglobulin in the presence of the sibling's T4+ cells. In contrast to the deficient inducer cells, the patient's T5+ T cells were capable of expressing suppressor-cell functions. These results indicate that immunodeficiency may occur because of a selective loss of T4+ inducer function.