Exposure of HeLa cells to 0.2 mM O6-methylguanine for 4 h or longer led to a 70-80% loss in the activity of the DNA-repair protein, O6-alkylguanine-DNA alkyltransferase. The decline in alkyltransferase activity brought about by O6-methylguanine was reversible on removing the base but at least 48 h were required for complete restoration. This loss of activity could also be brought about by other O6-alkylguanines including ethyl, n-propyl, and n-butyl, but the isopropyl and 2-hydroxyethyl derivatives were considerably less active. The rate of decline of alkyltransferase activity produced by O6-methylguanine was much more rapid than the rate of loss when protein synthesis was inhibited indicating that it was not brought about by blocking the synthesis of the protein. The loss of alkyltransferase activity was not prevented by the addition of inhibitors of nucleic acid or protein synthesis suggesting that it did not require protein synthesis or the incorporation of the O6-alkylguanine into nucleic acids. When cell free O6-alkylguanine-DNA alkyltransferase preparations were incubated in vitro with O6-methylguanine they became inactivated and when O6-[3H]methylguanine was used, [3H]guanine was produced. The inactivation was concentration dependent requiring 0.4 mM for a maximal rate and was quite slow requiring 3-4 hours for completion. These results suggest that the loss of O6-alkylguanine-DNA alkyltransferase activity is brought about by the free base acting as a very weak substrate for the protein. Exposure of mammalian cells to O6-methylguanine or O6-n-butylguanine provides a means by which the level of O6-alkylguanine-DNA-alkyltransferase can be regulated experimentally. This should enable the design of experiments to examine the role of O6-alkylguanine adducts in mutagenesis, carcinogenesis, and cell toxicity after administration of alkylating agents.