A method is presented to screen chemicals for potential mutagenicity on the basis of their ability to cause more killing in cells of repair-deficient yeast than in wild type cells. Two species were chosen in the event that one might be more sensitive to certain chemicals. The strains used were RAD+ and rad6 derivatives of Saccharomyces cerevisiae and RAD+ and rad3 derivatives of Schizosaccharomyces pombe. This report describes the test system and results for 12 known, direct-acting mutagens (i.e., not requiring mammalian metabolic activation). These compounds showed more lethality in one or both of the repair-deficient strains, indicating that they induce damage to DNA which is subject to repair in wild type cells. Advantages of this system include the use of eukaryotic yeast cells which can be manipulated as easily as bacteria, and that exogenous enzymes (S9) can be added for metabolic activation. Growing yeast cells can activate certain promutagens, and preliminary experiments showed positive responses for diethylnitrosamine and 2-acetylaminofluorene without the addition of S9.