Deoxyadenosine and its nucleotides have been implicated in the pathogenesis of the immune dysfunction associated with a genetic deficiency of adenosine deaminase (ADA). We have previously shown that when ADA is blocked with a synthetic inhibitor, human T lymphoblastoid cell lines are more sensitive to deoxyadenosine toxicity, dephosphorylate deoxyadenosine nucleotides at a slower rate, and have much lower levels of ecto-5'-nucleotidase than most B cell lines. It seemed unlikely, however, that an enzyme on the outer surface of the lymphocyte plasma membrane could regulate intracellular deoxynucleotide catabolism. We now report that human lymphoblasts also contain a soluble deoxynucleotidase activity that is distinguishable from the plasma membrane enzyme by several criteria. In multiple human lymphoblastoid cell lines of varying origin and phenotype. soluble deoxynucleotidase correlated significantly (rs = 0.80, p < 0.001) with sensitivity to deoxyadenosine toxicity.