To ascertain differences between solely hormone- and chemical carcinogen-induced murine mammary gland tumors (MGTs), a direct comparison of their ploidy status was assessed. Nuclear image cytometry (NIC) was used to evaluate ploidy in ductal carcinoma in situ (DCIS) and MGTs induced solely by 17-estradiol (E2) in female A-strain Copenhagen Irish hooded gene rats (ACI) and E2 plus testosterone propionate in male Noble rats. These results were compared