Fifty-nine canine mammary tumors have been simultaneously assayed for their histological nature and their content of cytosolic receptors for estradiol (ER), progesterone (PGR), and dihydrotestosterone (DHTR). The tumors were histologically defined as benign tumors, malignant mixed tumors, sarcomas, and simple or complex carcinomas. The tumors exhibited a high incidence of steroid receptors (ER in 61% of the tumors, PGR in 69%, DHTR in 36%). It could be demonstrated that, in cytosols of canine mammary tumors, binding sites for different steroids may simultaneously occur. Twenty four percent of the tumors were able to bind specifically all three hormones tested. No tumor class displayed a specific receptor profile in regard to the receptor incidence, KD, and binding capacity. Also no correlation could be detected between histological grading of carcinomas and receptor incidence. In these tumors, however, the amount of ER and PGR binding showed changes dependent on differentiation. Noteworthy tumors simultaneously removed from the same bitch and with identical histological diagnosis were different in their receptor characteristics.