Lac⁺ reversion assay with the E. coli strains WP3101P-WP3106P, derivatives of WP2 uvrA /pKM101, have been used to determine mutational specificity of mutagens. A minimal glucose medium supplemented with tryptophan (MGT) is used as a preculture medium in the Lac⁺ reversion assay. Nutrient broth (NB), on the other hand, is usually used for Trp⁺ reversion assay with the strain WP2 uvrA /pKM101. To improve the test procedure using a common preculture medium, we investigated the sensitivity of tester strains WP3101P-WP3106P against 11 kinds of mutagens when they were precultured in NB or MGT medium. In the Lac⁺ reversion assay, cells grown in MGT were as sensitive to MNNG, ENNG, EMS, and N⁴ -aminocytidine as those grown in NB. However, mutagenicities of MMS, captan, 5-diazouracil, and Na-azide were scarcely detected with NB-cultured cells. In contrast, cells grown in MGT were less sensitive to several mutagens in the Trp⁺ reversion assay than cells grown in NB. Therefore the use of a common preculture medium is not recommended for the Lac⁺ and Trp⁺ reversion assays.