Metalloestrogens are a new class of potent environmental estrogens. This study investigates whether tobacco smoke condensate (TSC), which contains metals and metalloids, elicits estrogen like effects at environmentally relevant doses. Treatment of human breast cancer cells, MCF-7, with 40 µg/ml of TSC resulted in a 2.5-fold stimulation of cell growth. TSC decreased the concentration of ER- protein and mRNA (63% and 62%), and increased the expression of the estrogen regulated genes, progesterone receptor (PR) and pS2 (5- and 2-fold). Additionally, TSC activated ER- in COS-1 or CHO cells transiently transfected with wild type ER- and an ERE-CAT or an ERE-luciferase reporter gene (11- and 6-fold). TSC also activated a chimeric receptor (GAL-ER) containing the hormone binding domain of ER- (3.5-fold). TSC blocked the binding of estradiol to the receptor without altering the affinity of estradiol (K_d = 2.2-6.8 x 10^-10 M). Transfection assays with ER- mutants identified C381, C447, H524, N532, E523, and D538 in the hormone binding domain as important for activation by TSC. In ovariectomized rats, low doses of TSC (10 or 20 mg/kg body weight) increased uterine wet weight (1.7- and 2.1-fold) and induced the expression of PR and complement C3 in the uterus (2- and 26-fold) and mammary gland (4.4- and 15-fold). Both the in vitro and in vivo TSC effects were blocked by the antiestrogen ICI 182,780 suggesting the involvement of ER. Collectively, these results provide strong evidence that low doses of TSC, acting through the hormone binding domain, exert estrogen like effects in cell culture and in animals.