Mammary tumour tissue from two bitches and an anal adenoma from a dog were investigated for steroid receptor interaction. Both mammary tumours possessed cytoplasmic macromolecules sedimenting with coefficients of 4S and 8S that bound oestradiol-17beta. These receptors had molecular weights of approximately 60,000 and 180,000 respectively. Transfer of the oestrogen to the nucleus was shown and the presence of a 4-5S nuclear protein demonstrated. The anal adenoma had a cytoplasmic receptor, with a sedimentation value in a sucrose density gradient of 4-5S with respect to bovine serum albumin, that bound tritiated 5alpha-androstane-3alpha, 17alpha-diol. No affinity could be demonstrated for other C19-steroids examined. The significance of these findings in terms of the hormone dependence of the tumours investigated and the possible development of these studies to promote rational therapy in such cases is discussed.