Adenosine and deoxyadenosine toxicity was examined in colony assay systems for human T lymphocytes, B lymphocytes, and granulocytes. In the absence of deoxycoformycin, an adenosine deaminase inhibitor, no growth inhibition was observed in the three systems with concentrations of adenosine or deoxyadenosine of at least 200 microM. Deoxycoformycin itself had no growth-inhibitory effect at concentrations of at least 10 micrograms/ml. Combinations of deoxycoformycin (1 microgram/ml) and either adenosine or deoxyadenosine gave growth inhibition in all three systems. Deoxyadenosine was the most toxic in all the systems, the LD50 values being 20-25 microM. The LD50 values for adenosine were 45-55 microM. There was no evidence of selective toxicity by adenosine or deoxyadenosine with these three colony assay systems. In the T-lymphocyte colony system deoxyadenosine appeared to be toxic to both the inducer/helper and the suppressor/cytotoxic T-lymphocyte subpopulations.