We have previously reported that stromal fibroblasts of ovarian carcinoma specimens may express estrogen (ER) and progesterone receptors (PR) when the malignant epithelial cells do not, and even when the specimens have been obtained from such non-Müllerian structures as the omentum whose fibroblasts normally express neither ER nor PR. In an attempt to investigate whether our observations of the expression of ER and PR in fibroblasts surrounding metastatic invasive epithelial ovarian carcinoma cells might result from an interaction involving malignant epithelial cells and stromal fibroblasts, we co-cultivated in vitro BG1 ovarian carcinoma cells with sex steroid receptor-negative dermal fibroblasts to determine whether carcinoma cells might induce the latter to express ER or PR protein and transcripts at levels detectable by standard immunocytochemical (ICC) and in situ hybridization (ISH) techniques. We report the in vitro induction of ER and PR transcripts and protein in previously steroid receptor-negative skin fibroblasts after co-cultivation with BG1 ovarian adenocarcinoma cells. Such observations suggest that a juxtacrine mechanism is responsible for the observed phenomenon, possibly involving ER- and PR-inducing factors (ER-IF and PR-IF).