Abstract Serial-flow cytometric analysis of DNA content of T lymphoblasts (MOLT-4) and B lymphoblasts (MGL-8) was performed to correlate the cytotoxic properties of adenosine deaminase inhibition with alterations of DNA synthesis and disruptions of the cell cycle. The addition of deoxyadenosine up to 50 μmol/L potently decreased the growth of T lymphoblasts, and these changes were enhanced with the addition of 100 μmol/L homocysteine thiolactone. These conditions caused a virtual absence of cells from S and G 2M phases after 24 hours. The DNA distribution was similar in cells cultured for 24 hours in 50 μmol/L deoxyguanosine or 2.5 μmol/L hydroxyurea. These observations suggested accumulation of cells in the G 1 phase. T lymphoblasts cultured with up to 50 μmol/L adenosine had a substantial decrease in growth, which was not modified by the addition of homocysteine thiolactone. Cell cycle distributions of T lymphoblasts cultured for 24 to 48 hours under these conditions showed mild decreases in the G 2M population. The addition of adenosine up to 50 μmol/L decreased the growth of B lymphoblasts, and these changes were enhanced by the addition of 100 μmol/L homocysteine thiolactone. These conditions induced mild decreases in the S-phase population in B lymphoblasts. The addition of deoxyadenosine, even with homocysteine thiolactone, did not modify growth in B lymphoblasts and the cell-cycle distributions were indistinguishable from distributions of control populations after 24 and 48 hours. The observations provide independent support for a reduction of DNA synthesis associated with cytotoxicity during adenosine-deaminase inhibition. These changes were more profound in T lymphoblasts than B lymphoblasts. Cytotoxicity during S and G 2M phases and/or a G 1 S transition block could account for the cell-cycle alterations observed.