dATP, dADP, and dAMP equalled or exceeded the depleted levels of ATP, ADP, and AMP in erythrocytes from two children with adenosine deaminase (ADA; EC 3.5.4.4) deficiency. dATP and dADP were identified in the mononuclear cells of only one child. The levels of deoxyadenosine compounds fell dramatically after enzyme replacement therapy and were no longer detectable in the urine or in mononuclear cells. Erythrocyte adenosine nucleotide levels showed a corresponding increase. Intact erythrocytes prior to treatment contained adenine, presumed to be from deoxyadenosine degraded during extraction. Adenosine at high concentrations in vitro increased both dATP and ATP levels and decreased intracellular deoxyadenosine levels. There was no significant deamination of either [8-14C]adenosine or deoxyadenosine by intact ADA-deficient erythrocytes. About 90% of adenosine was metabolized to ATP at substrate concentrations from 10-100 microM, compared to 40-60% of deoxyadenosine metabolized to dATP. These studies suggest that (i) high intracellular deoxyadenosine levels may be necessary in vivo to sustain the raised dATP levels in ADA deficiency. (ii) When ADA is inhibited or absent, deoxyadenosine is removed rapidly from the circulation by the human erythrocyte utilizing an adenosine transport system linked to both ADA and adenosine kinase (EC 2.7.1.20).