The feasibility of using radioiodinated prolactin for early detection of solid mammary tumors was studied in the rat. Radioiodinated prolactin was first injected into tumored and control rats in order to define the localization of the labeled hormone in their target organs, then it was injected into extracorporeal blood-loop of tumored and healthy rats, in order to study its clearance in those animals. At 15 min post injection, 75-82% of the labeled hormone was recovered, of which 4-5% were counted in the tumor. Deiodination was apparent only 30 min after injection. As much as high deposition of the labeled hormone was not achieved in the hormone-dependent tumor lines investigated, some of the pitfalls that may be encountered are documented. The clearance of the labeled prolactin as determined by the blood-loop technique is 3.3 min in the R3230 tumor, and 6.4 min in the 13762 mammary tumor. The healthy rats had a blood clearance of 4.6 min.