Histochemical studies were performed with the labeled lectin from Arachis hypogaea (peanut agglutinin, PNA) to detect the occurrence, distribution, and amount of this lectin receptor in N-nitrosomethylurea-induced rat mammary carcinomas. Since in normal rat mammary tissue, the formation and secretion of PNA binding sites are controlled by the combined action of estrogen and prolactin, the amount of secretion-associated PNA binding sites was correlated with tumor response to either ovariectomy or antiestrogenic treatment with tamoxifen. Free as well as sialic acid-substituted PNA binding sites which were represented by the disaccharide beta-D-Gal(1-3)NAcGal occurred in the apical cytoplasm of tumor cells and within the secretions of acinar and tubular structures. In contrast the whole tumor cell membrane contained predominantly sialic acid-substituted PNA binding sites. All of the tumors investigated contained some secretion-associated PNA binding sites. The values showed a continuum from very low to very high amounts. The number of PNA binding sites associated with secretory activity was lower in hormone-independent than in hormone-dependent tumors. Furthermore, there was an inverse relationship between mitotic and secretory activities with positive PNA staining. In conclusion, these studies revealed that the secretory PNA index, mitotic index, and especially the ratio of secretory PNA index to mitotic index represent useful parameters to select tumors for endocrine therapy in this experimental model.