By a sensitive enzyme-linked immunosorbent assay, inactive variant nucleoside phosphorylase (NP) protein could be quantitated in red cells and cultured skin fibroblasts from the probands and parents in four families with enzyme-deficient members. Three different mutant alleles could be identified that, in the homozygous state, could lead to T-cell immunodeficiency. The mutant alleles formed proteins that differed from normal NP protein by their stability, catalytic activity, or isoelectric charge. Thus, sensitive biochemical techniques can demonstrate the presence of different mutations for purine NP. Any of the mutations can be responsible for a lack of purine NP and the development of the characteristic T-cell deficiency syndrome.