Spontaneous mutant frequency in livers of two transgenic mouse strains, each carrying identical lambda shuttle vectors with a lacI target gene, was evaluated by two laboratories. These studies investigated variability in spontaneous mutant frequency between animals and as a function of the number of phage screened. Liver DNA was independently isolated from 7-11 week old C57BL/6 and B6C3F1 Big Blue transgenic mice. At least 500,000 phage were screened for mutation at lacI for each animal using standardized assay procedures. In the two labs, the C57BL/6 liver spontaneous mutant frequency was 45 +/- 9 x 10(-6) and 41 +/- 7 x 10(-6). The B6C3F1 liver spontaneous mutant frequency was 42 +/- 10 x 10(-6) at one lab and 43 +/- 12 x 10(-6) and 41 +/- 8 x 10(-6) in two trials at the second lab. Mean mutant frequency data from both labs, calculated in increments of 100,000 plaque forming units (pfu) scored for each mouse strain, show stabilized mean mutant frequency and standard deviation after approximately 200,000-300,000 pfu screened. The frequency of spontaneous lacI mutants was reproducible both within and between labs and was comparable between the two transgenic mouse strains.