Neonatal treatment of female rodents with the synthetic estrogen diethylstilbestrol [(DES) CAS: 56-53-1; alpha, alpha'-diethyl-4,4'-dimethoxystilbene] results in immunodeficiency that persists into adulthood, along with progressive development of genital tract lesions. DES-induced immunosuppression was examined as a possible promoter of genital tract lesion and tumor development in BALB/cCrgl female mice. Secondary suppression of T-cell immunity after neonatal DES treatment failed to increase lesion incidence or to promote genital tract tumor development in 8-month-old mice. Although hyperplastic lesions were present in 70% of the genital tracts from DES-treated mice at 8 months of age, apparently few neoplastic cells are present at this time, since transplantation of genital tracts from these animals into syngeneic hosts did not yield tumors. To reduce the possibility that potential tumors were too immunogenic to survive in the syngeneic hosts, genital tract pieces from 12-month-old DES-treated female mice were transplanted into immunosuppressed hosts; only 1 tumor resulted. The tumor was a mixed adenocarcinoma, squamous cell carcinoma, and sarcoma and was immunogenic. These results suggest that immunosurveillance by T-cells is relatively unimportant in DES-induced genital tract lesion development.