Polyclonal antibodies were raised in rabbits by immunization with free peptides corresponding to positions 197-222 of the human beta 1-adrenergic receptor (beta 1 peptide) and the corresponding sequence (172-197) of the human beta 2-adrenergic receptor (beta 2 peptide). While the beta 2 peptide yielded antibodies that cross-reacted with the beta 1 peptide, the antibodies against the beta 1 peptide did not cross-react with the beta 2 sequence. Cross-reactivity of the anti-beta 2 peptide antibodies and the selectivity of the anti-beta 1 peptide antibodies were also revealed in the recognition by immunoblots of the beta 1- and beta 2-adrenergic receptors of different species or of the receptor gene products expressed in a bacterial vector. These antibodies could be used immunohistochemically to visualize the beta-adrenergic receptors on rabbit heart. The anti-beta 2 peptide antibodies did not show any functional effect on the beta-adrenergic receptors; the anti-beta 1 peptide antibodies were able to displace agonist affinity to higher values. Recognition of truncated peptides by the anti-beta 1 and anti-beta 2 peptide antibodies suggested that the cross-reaction of the anti-beta 2 peptide antibodies was due to the recognition of a common epitope on the C-terminal part of the peptides. The anti-beta 1 peptide antibodies recognized the N-terminal part of the peptide better than the C-terminal part. These results suggest that the second extracellular loop postulated in the structure of the human beta-adrenergic receptor contains the T and B cell epitopes necessary for induction of an immune response. The selectivity and the functional properties of the antibodies raised against that loop in the beta 1 adrenergic receptor could have relevance in induction of auto antibodies in certain cardiomyopathic conditions.