AbstractThe variant frequencies for 6‐thioguanine‐resistant spleen cells in different mouse strains have been estimated by autoradiography for animals without chemical treatment and in cases of in vivo mutagen dosage with ethylnitrosourea and cyclophosphamide, respectively. In untreated mice, the following variant frequencies have been found: C57B1/6J, 2.84 × 10−5;NMRI, 3.04 × 10−5;DBA/2J, 5.91 × 10−5. The selective concentration of 6‐thioguanine was 100 μM for strains NMRI and DBA, while in the case of C57B1 with this concentration, no variant cells could be counted and a selective concentration of 50 μM was chosen. Treatment with 70, 140, and 210 mg/kg ethylnitrosourea resulted in increased variant frequencies in cells isolated 8 or 15 days later. On the other hand, doses of 20, 60, and 120 mg/kg cyclophosphamide did not result in a clear dose‐response relationship of variant frequency in cells isolated 1, 8, 15, 22, and 29 days after treatment. These data are discussed with respect to findings in human populations exposed occupationally t