Certain forms of severe immunodeficiency syndrome (SCIDS) are related to adenosine deaminase deficiency in lymphocytes. This impairment in lymphocyte metabolism leads to an accumulation of purine nucleotides that inhibit normal lymphocyte function. Diagnosis of this variant form of SCIDS is facilitated because the same enzyme defect occurs in the patient’s red blood cells. In addition, another immunodeficiency syndrome due to a selective defect in T-cell function is related to a deficiency of purine nucleoside phosphorylase in lymphocytes. This defect can also be detected in erythrocytes.