The cumulative effects of Lentinan and endocrine therapy on the growth of DMBA (7, 12-dimethyl benzanthracene)-induced mammary tumors of rats were studied. Multiple injection of Lentinan alone resulted in a slight degree of regression of the tumor growth, when administered to rats bearing mammary tumors of about 1 cm in size. Ovariectomy-adrenalectomy, ovariectomy, and adrenalectomy, which are performed as surgical endocrine therapy, resulted in a more marked regression of the tumor than that produced by Lentinan treatment alone. Furthermore, multiple injection of Lentinan performed on these mammary tumor-bearing rats which had received surgical endocrine therapy 2 weeks previously evoked a marked regression of tumor growth. However, no changes in growth curves and survival rates, compared with those of saline controls were observed, indicating that Lentinan might be a useful agent when combined with surgical endocrine therapy. Concurrent injection of Lentinan resulted in a slight augmentation, although the histamine sensitivity of tumor bearing rats which had previously received surgical endocrine therapy elevated greatly, compared with that of controls. By contrast administration of Tamoxifen, which is used for medical endocrine therapy, resulted in a lesser degree of regression than that observed following surgical endocrine therapy, and was also not greatly affected by Lentinan injection.