A monoclonal antibody was raised against the affinity purified beta 1-adrenergic receptor from turkey erythrocytes. This antibody, B120, of the IgG1 isotype, precipitates the photoaffinity-labeled purified receptor and the corresponding binding activity. The antibody recognizes the 42 kDa component of the receptor of erythrocyte membranes, after electrotransfer on nitrocellulose. B120 does not inhibit binding of dihydroalprenolol on membranes and has no effect on the activity of adenylate cyclase. Since specific immunofluorescence was detected only after ethanol treatment of the erythrocytes, the epitope recognized by B120 appears not to be accessible at the cell surface.