Selective agonist of nonopioid beta-endorphin receptor decapeptide immunorphin (SLTCLVKGFY) was labeled with tritium (the specific activity of 24 Ci/mmol). [3H]Immunorphin was found to bind to nonopioid beta-endorphin receptor of mouse peritoneal macrophages (Kd = 2.0 +/- 0.1 nM). The [3H]immunorphin specific binding with macrophages was inhibited by unlabeled beta-endorphin (Ki = 2.9 +/- 0.2 nM) and was not inhibited by unlabeled naloxone, alpha-endorphin, gamma-endorphin and [Met5]enkephalin (Ki > 10 microM). Thirty fragments of beta-endorphin have been synthesized and their ability to inhibit the [3H]immunorphin specific binding to macrophages was studied. Unlabeled fragment 12-19 (TPLVTLFK, the author's name of the peptide octarphin) was found to be the shortest peptide possessing practically the same inhibitory activity as beta-endorphin (Ki = 3.1 +/- 0.3 nM). The peptide octarphin was labeled with tritium (the specific activity of 28 Ci/mmol). [3H]Octarphin was found to bind to macrophages with high affinity (Kd = 2.3 +/- 0.2 nM). The specific binding of [3H]octarphin was inhibited by unlabeled immunorphin and beta-endorphin (Ki = 2.4 +/- 0.2 and 2.7 +/- 0.2 nM, respectively).