Prolactin receptors have been measured in 92 human breast carcinomas. Both free and total receptors (after desaturation by MgCl2) have been looked for. Free receptors have been found in 46% of the cases, total receptors in 72%. Specific binding ranges from 0.8 to 8.0%. No correlation could be found between prolactin receptors and age, weight, menopausal status and pathological features (differentiation, histoprognostic grading, cellular density). A highly significant correlation has been found between prolactin receptors on the one hand and estradiol and progesterone receptors on the other.