A soluble somatostatin-binding protein was detected in the cytosol fractions of various rat, human and bovine tissues. Maximum binding occurred at pH8.0–8.5 and was Ca 2+ -dependent. The specific binding of somatostatin per 10μg of cytosol protein from 12 rat tissues ranged between 36 and 15%, and 3% for peripheral blood cells. There was also substantial binding in cytosol from human anterior pituitary and liver, and bovine anterior pituitary. The specific binding in rat and human plasma in the presence of EDTA was only 1%. Gel filtration suggested a molecular weight of approx. 80000 for the somatostatin-binding protein from several sources. Exposure of the binding protein to trypsin eliminates somatostatin-binding activity but ribonuclease and deoxyribonuclease have no effect. The binding protein is thermolabile, ethanol-precipitable, and not completely specific for somatostatin. Bound 125 I-labelled [Tyr 1 ]somatostatin is not easily displaced by excess of unlabelled somatostatin. The effects of dithiothreitol and mercaptoethanol on the binding of 125 I-labelled [Tyr 1 ]somatostatin to the binding protein suggests that binding involves two sequential steps, first loose binding, then disulphide linkage. Since semipurified somatostatin-binding protein causes a dose-related inhibition of the binding of 125 I-labelled [Tyr 1 ]somatostatin in radioimmunoassays for somatostatin, estimates of somatostatin content of tissue extracts by radioimmunoassay in some cases may be spuriously high. It is not yet clear whether the binding protein is a true cytosol protein or an easily solubilized membrane protein.