A method is described for curing the Ames Salmonella mutagen tester strains of their Fels 1 and Fels 2 prophages with the aid of the antitumor drug daunorubicin. Non-lysogenic derivatives corresponding to TA100 and TA1535 were isolated and designated TAQ100 and TAQ1535 respectively. In addition, the Fels 1 monolysogens TAQ100F1 and TAQ1535F1, as well as the Fels 2 monolysogens TAQ100F2 and TAQ1535F2, were obtained. Finally, strains corresponding to TA98 and TA1538 cured of Fels 2, but retaining a cryptic Fels 1 (F1d) prophage were isolated and designated TAQ98F1d and TAQ1538F1d respectively. The various cured derivatives were identified by colony hybridization with 32P-labeled probes of Fels 1 and Fels 2 DNA. Southern blot hybridizations confirmed that phage-specific Fels DNA sequences were missing from the cured strains. The Fels 2-cured strains were resistant to Fels 2, but Fels 1 grew, albeit poorly, on the Fels 1-cured strains. Strains TAQ100F1, TAQ1535F1, TAQ100F2 and TAQ1535F2 were used in prophage induction assays, in the presence of rat-liver extract where necessary. Daunorubicin, bleomycin, mitomycin C, aflatoxin B1, 2-amino-dipyrido[1,2-a:3',2'-d]imidazole (Glu-P-2) were found to induce Fels 1 and/or Fels 2 in at least one of these strains. The induction of the Fels prophages in the TAQ monolysogens may provide a useful complement to the Ames test for the detection of DNA-damaging agents and potential carcinogens.