The DNA repair enzyme O6-alkylguanine-DNA alkyltransferase (AT) was analysed in the human ovarian-cancer SW626 cell line and in the human promonocytic leukemia U937 cell line following their synchronisation with low non-toxic concentrations of methotrexate. In SW626, AT increased in the early S phase of the cell cycle and then declined during progression of the S phase to levels found in the G1 phase of unsynchronised cells. In contrast, at the G1/S-phase boundary and in the S phase, U937 cells showed a lower AT content than did exponentially growing unsynchronised cells. In addition, AT activity was greatly reduced in resting U937 cells but was not reduced appreciably in resting SW937 cells but was not reduced appreciably in resting SW626 cells. The results of these studies indicate that AT fluctuations do not follow a constant pattern during the cell cycle of different cell lines.