Prostaglandin F2 alpha receptors (PGF2 alpha Rs) were measured in bovine corpus luteum and myometrial cell membranes using a radiometric method. The inhibition of labelled PGF2 alpha binding exerted by d-cloprostenol, dl-cloprostenol, PGF2 alpha and PGE1 (10(-11) M to 10(-4) M) was evaluated in vitro. Results strongly suggest that cloprostenol binding to PGF2 alpha Rs is stereospecific. d-Cloprostenol and PGF2 alpha were equipotent, about 150 times more potent than dl-cloprostenol (P < 0.05) and approximately 280 times more potent than PGE1 (P < 0.05) in inhibiting [3H]PGF2 alpha binding to corpus luteum cell membranes. Such differences were less evident in myometrial cell membranes, where d-cloprostenol and PGF2 alpha were about 10 times more potent than dl-cloprostenol (P < 0.05) and approximately 95 times more potent than PGE1 (P < 0.05).