Activities of enzymes of the purine metabolic pathway, adenosine deaminase (ADA), purine nucleoside phosphorylase (PNP), and 5′- nucleotidase (5′-N), were investigated in the lymphoblasts of a patient with B-cell acute lymphoblastic leukemia. These lymphoblasts exhibited increased ADA activity and diminished activities of both PNP and 5′N' as compared to normal lymphocytes as well as non-T, non-B leukemia cells. This enzymatic pattern is identical to that which has been described in T-cell leukemic lymphoblasts and differs from that which has been observed in the malignant cells of undifferentiated B-cell lymphomas. These data suggest that there is biochemical heterogeneity within the spectrum of B-cell malignancies. Furthermore, inhibitors of ADA may be of use in those B-cell lymphoid neoplasms that exhibit increased ADA activity.