The recent introduction of the phenyl-β-D-galactopyranoside (P-gal)-based positive-selection system for screening of λlacZ phages originating from the λlacZ transgenic mouse (Muta Mouse) has made the determination of mutant frequencies (MF) a much simpler task. Previously, MF data from these mice have been collected by means of the 5-bromo-4-chloro-3- indolyl-β-D-galactopyranoside (X-gal) colour-screening procedure. To determine whether data obtained with the two systems are comparable, the MF in h phages recovered from liver and brain of transgenic mice treated with N-ethyl-N-nitrosourea (ENU) and liver of benzo(a)pyrene (B(α)P)-treated mice was determined with both procedures. For the livers of mice treated with ENU, both methods yielded approximately the same MF values. No induction of mutants, relative to the control animals, was seen after 1.5 h, but a clear 4-fold increase was measured with both assays at the 14-day time point. No induction of mutants was found in the brain with either method. In the B(α)P-treated mice, both methods showed a substantial induction in MF after 21, 28 and 35 days. The values generated by the X-gal and P-gal methods were not significantly different, with the exception of the 35-day post-treatment point that appeared higher in the X-gal assay. When the mutants isolated by use of the X-gal method were tested in the P-gal assay, a number of these did not turn up as mutants, and the significance disappeared, In conclusion, the data obtained with the two screening procedures agree to such an extent as to permit a direct comparison between the earlier results generated with X-gal and P-gal values generated with the new positive-selection method. This is likely to apply also to other organs and mutagens than those studied here.