Biopsy specimens of 55 human mammary carcinomas (38 primary and 17 metastatic) were assayed for prolactin receptors (PrlR). Prolactin bound specifically to 32 (58%) of the tumor biopsy specimens. The apparent Kd for PrlR in individual tumors ranged from 15 pM to 2.3 nM (mean 600 pM, n = 5) and the concentration of PrlR ranged from 0 to 44.5 fmoles/mg protein. Estrogen receptors (ERP) were also detected in 28 of the 32 tumors which had PrlR. Overall, there was no correlation between PrlR and ERP. However, the mean concentration of PrlR was significantly higher (p less than 0.01) in tumors with 6-100 fmoles/mg protein ERP (approximately 13 fmoles PrlR) than in tumors with either less than 6 or greater than 250 fmoles ERP (4.0 +/- 0.4 and 6.5 +/- 1.8 respectively fmoles PrlR). Analysis of PrlR concentration as a function of patient age also showed no overall correlation, but the mean PrlR in tumors from women aged 60-70 was significantly higher (p less than 0.01) than in those from either younger or older women. A higher concentration of PrlR was observed in tumors which were classified histologically as medium or well differentiated (6.1 +/- 1.2 and 11.1 +/- 2.1, respectively) than in those classified as poorly differentiated (3.3 +/- 1.2) (p less than 0.03). There was a negative correlation between PrlR concentration and membrane yield from the tumors (r = 0.43, p less than 0.02). The membrane yield correlated with the ratio of tumor cells to stroma (histologically) (r = 0.63, p less than 0.001). In tumors from 12 patients with metastatic disease on whom follow up after endocrine-related therapy was available, the mean PrlR concentration was significantly higher in the non-responding group (8.2 +/- 3.0) than in the responding group (3.4 +/- 4.2, p = 0.05).