Tetrahymena, a ciliated protozoan, is a highly specialized, differentiated eukaryotic organism. It is known to possess many informational substances, including beta-endorphin (beta E). We wished to investigate the possibility that this organism possesses a functional opiate receptor which might be similar to the well-characterized opiate receptor in the rat brain. Binding assays using both living cells and membrane preparations, verified stereospecific, saturable, reversible 125I-beta E binding. This binding was displaceable by various opiates chosen to represent each of the putative opiate subtypes. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of a disuccinimidyl suberate cross-linked 125I-beta E-receptor complex revealed a pattern of bands which consistently included bands at 110, 58-55, and 29 kDa. These bands, which were all displaceable by the classical antagonist, naloxone, as well as by other opiates, are thought to be prototypic for various opiate receptor subtypes. Limited proteolysis in SDS-PAGE showed that the 110 kDa band could be fragmented into 58-55 and 29 kDa bands and that the 58 kDa band could generate a 29 kDa fragment. The limited digest fragments of the 110, 58-55 doublet and 29 kDa bands were remarkably similar to those generated from the rat brain receptor. Analytical isoelectric focusing of digitonin solubilized 125I-beta E-receptor complexes showed the isoelectric points (pI) from both the rat and Tetrahymena were identical (pI 4.6). Chemotactic experiments with the intact Tetrahymena, demonstrated that these unicellular animals migrated toward a 10(-9) M beta E gradient. Chemotaxis was blocked by (-)-naloxone but not (+)-naloxone, suggesting a stereospecific opiate receptor-mediated response. We conclude that Tetrahymena possesses a functional opiate receptor (recognition molecule) very similar to the opiate receptor of the rat brain.