The cholecystokinin (CCK) receptor in purified plasma membranes prepared from mouse pancreatic acini had a binding affinity of 1.8 nM, an acid pH optimum between 6.0 and 6.5, and an analog specificity of CCK 8 > CCK 3 3 > desulphated CCK 8 > CCK 4 . Binding of CCK to its receptor was abolished by pretreatment of plasma membranes with trypsin. When [ 1 2 5 I]CCK was cross-linked to its receptors with disuccinimidyl suberate, and the preparation solubilized and subjected to gel electrophoresis and autoradiography, the hormone was associated with M r 80000 protein in both the presence and absence of the reducing agent dithiothretol.