In this paper we examine the binding of [3H]FOXY (tritiated-6-beta-fluoro-6-desoxy-oxymorphone) to membranes of rat brain. Using the site-directed alkylating agents BIT and FIT, evidence is presented that [3H]FOXY selectively labels mu opiate binding sites in vitro. Further, BIT and FIT did not significantly affect [3H]bremazocine binding to kappa receptors. Scatchard plots of [3H]FOXY binding were somewhat curvilinear, suggesting the presence of two classes of mu binding sites. At concentrations up to 19 nM, 90 percent of the total binding was specific. The combination of high mu-selectivity and low nonspecific binding suggests the [3H]FOXY may prove to be a powerful tool for studying the opiate mu receptor.