Patients with primary disorders of B lymphocytes and immunoglobulins usually display increased susceptibility to bacterial infections but atopic, autoimmune, and malignant disorders are also more common in these patients. The spectrum of these disorders ranges from a virtual absence of B cells and immunoglobulins to selective deficiencies of immunoglobulin subclasses. The diagnosis is dependent upon the demonstration of the immunologic deficits by specialized laboratory procedures which include the quantitation of immunoglobulins, the formation of antibodies in vivo and in vitro and the demonstration of B cells in the tissues or the peripheral blood. There are five major points in the management of these patients: (1) the delineation of the immunologic defects by laboratory testing, (2) the use of parenterally injected human immunoglobulins, (3) the rapid identification of infecting organisms and the prompt institution of appropriate antibiotic therapy, (4) screening the family for immunodeficiency in those cases which are of genetic origin, and (5) genetic counseling.