The levels of adenine phosphoribosyltransferase (APRT:EC 2.4.2.7) were determined in red blood cells (RBCs), peripheral mononuclear cells (MNCs), and polymorphonuclear leukocytes (PMNLs) from normal controls and from two families with APRT deficiency. No APRT activity was demonstrated in MNCs and PMNLs of patients with complete deficiency of RBC-APRT. APRT deficiency occurs not only in RBCs but also in MNCs and PMNLs. Immunologic and phagocytic examinations showed normal hemogram and serum immunoglobulin levels, and normal E-rosette forming cells and surface immunoglobulin-bearing cells. Lymphocyte blastogenesis in response to phytohemagglutinin and lymphocyte differentiation to cytoplasmic immunoglobulin-producing cells induced by pokeweed mitogen were normal. No major defects were apparent in natural killer activity. Phagocytic functions were normal as tested by bactericidal activity, O2-consumption, chemotaxis, and chemiluminescence response.