Cryptic receptor sites in the guinea-pig ileum preparation have been uncovered by the treatment of the preparation with the highly selective, irreversible mu opioid receptor antagonist, beta-funaltrexamine. These beta-funaltrexamine-insensitive sites appear to interact only with opioid peptides ([D-Ala2, D-Leu5]enkephalin, [D-Ala2, Met5]enkephalinamide, Tyr-D-Ser-Gly-Phe-Leu-Thr and [D-Ala2, MePhe4, Gly-ol5]enkephalin) but not with nonpeptide agonists. These new sites could not be protected by either mu-selective (morphiceptin and [D-Ala2, MePhe4, Gly-ol5]enkephalin) or delta-selective ([D-Ala2, D-Leu5]enkephalin, Tyr-D-Ser-Gly-Phe-Leu-Thr, (Allyl)2-Tyr-Gly-Gly-psi-(CH2S)-Phe-Leu, and (Allyl)2-Tyr-Aib-Aib-Phe-Leu) peptides against beta-chlornaltrexamine alkylation. However, naloxone afforded full protection of these sites against beta-chlornaltrexamine alkylation. The delta-selective antagonist, (Allyl)2-Tyr-Aib-Aib-Phe-Leu, had no activity at these cryptic sites at concentrations that effectively blocked delta receptors in the mouse vas deferens. The cryptic sites do not appear to be typical mu or delta receptors. The new receptors were termed mu', a mu subtype, and a receptor model that is consonant with our data is presented.