Monoclonal antibodies were produced following immunization of mice with either [Leu5]enkephalin-bovine serum albumin or [Met5]enkephalin-keyhold limpet hemocyanin conjugates. Two monoclonal antibodies coded NOC1 and NOC2, respectively, were derived. These monoclonal antibodies did not discriminate between Leu- and Met-enkephalin in either radioimmunoassay or immunocytochemistry. NOC1 was characterized in detail. In radioimmunoassay NOC1 displayed about 40% crossreactivity with C-terminal extended Met-enkephalin hexapeptides and 7% with the extended heptapeptide (-Arg-Phe-OH), but did not recognize other endogenous peptides. In immunocytochemistry the NOC1 and NOC2 recognized all well-established "enkephalin immunoreactive sites," but they did not bind to areas known to contain beta-endorphin or high levels of pro-enkephalin. NOC1 was shown to be a suitable tool to demonstrate enkephalin immunoreactive sites by radioimmunocytochemistry utilizing both internally and externally labeled monoclonal antibodies.