Morphine and other opioids have been reported to modulate phagocytosis in the ciliate Tetrahymena. However, the endogenous signaling molecule responsible for these effects remains uncharacterized. In this work we present evidence for the presence of β-endorphin-like protein(s) in Tetrahymena thermophila. Subcellular extracts and cell-free culture supernatants were fractionated by hydrophobic chromatography on Sep Pack C₁₈ columns and by affinity chromatography on polyclonal anti-β-endorphin columns. Both preparations exhibited opioid-like effects in two different systems: 1) they inhibited phagocytosis in murine peritoneal macrophages, and 2) they blocked the response to mechanical stimuli in the ciliate Stentor. Both of these effects were reversed by naloxone, consistent with an opioid receptor-mediated mechanism. Chromatographic (HPLC) fractionation of the subcellular extracts resolved a component with β-endorphin-like immunoreactivity, whose retention time was similar to that of the human β-endorphin standard. Fractions were also analyzed by immunoblots using a monoclonal antibody that recognizes the N-terminus of human β-endorphin. This antibody detected two antigenic components (corresponding to Mr 9,000 and Mr 12,000 polypeptides) in subcellular extracts, but only a single antigen (corresponding to a Mr 7,000 polypeptide) in culture supernatants. These results indicate that Tetrahymena produces one or more proteins that share some properties with β-endorphin and that these may form part of an opioid mechanism that originated early in evolution.