Strong immunoreactivity with polyclonal S-100 protein antisera and monoclonal S-100 alpha subunit antiserum was found in glandular cells of the decidua basalis and cervical polyps during early pregnancy. Immunoreactive S-100 protein was negative in glandular cells of the endometrium and cervix of nonpregnant women. It was also negative in endometrial hyperplasia and endometrial carcinoma. While the function of S-100 protein is not known, a relationship between humoral factors related to pregnancy and expression of S-100 protein gene is suggested by the results of this study.