Specific binding of 125I-ovine prolactin was measured in mammary tumors from rats given 7, 12 dimethylbenzanthra-cene. A significant correlation was noted between the binding of prolactin to par-ticulate membrane fragments in the total microsomal pellet and the growth response of these tumors to the administration of prolactin, that is the tumors showing the greatest response to prolactin also bound the greatest amount of 125I-ovine prolactin (r = 0.69). There was a negative correlation between the average growth response of all tumors in one rat and the amount of 125I-ovine prolactin specifically bound to liver membranes of that animal (r = — 0.69). This or a similar method of determining prolactin dependence may be useful in assessing prolactin responsiveness in human breast cancer.