Purine nucleoside phosphorylase (PNP) activity has been measured in the lymphoblasts of 22 patients with acute lymphoblastic leukemia (ALL) and correlated with routine immunologic cell surface markers. Fourteen of the 22 patients were considered to have non-T, non-B-cell ALL; 8 patients had T-cell disease. The median PNP activity in 21 control samples of normal peripheral blood mononuclear cells was 83 U. The median PNP activity of the non-T, non-B lymphoblasts was 79 U. No statistical difference in PNP activity between these two groups could be discerned (p < 0.37). In contrast, T-cell lymphoblasts demonstrated diminished PNP activity with a median of 38 U. The differences in activity between T lymphoblasts and both non-T, non-B leukemic cells and normal peripheral blood mononuclear cells were significant (p < 0.001 and p < 0.003, respectively). Evaluation of PNP activity provides further evidence of biochemical heterogeneity among immunologic subclasses of ALL.