Peroxyacetic (PAA) and monoperoxydecanoic (MPDA) acids were assayed for induction of unscheduled DNA synthesis (UDS) by liquid scintillation counting of hot-acid-extractable DNA and by light microscope autoradiography. Both compounds were also assayed for induction of DNA repair synthesis by differential density labeling ultracentrifugation. Uniformly negative results were obtained for MPDA. Conflicting results were obtained for PAA using the UDS techniques. Negative results were consistently obtained, however, in three separate assays using two different PAA samples with the more definitive differential density DNA repair synthesis technique. Hydrogen peroxide, which is present as a contaminant in the PAA samples, was also assayed for induction of UDS by autoradiography and for induction of DNA repair synthesis by differential density labeling. Our results with this compound are in agreement with published data and were consistently positive using both techniques. We conclude that neither MPDA nor PAA induce DNA repair synthesis and suggest that the conflicting PAA results may be due to the presence of hydrogen peroxide in commercial samples of PAA.