Although opioid peptides such as methionine (met)-enkephalin have been previously shown to enhance or suppress immune responses, few studies in animal models have addressed the immunomodulatory activity of their metabolic derivatives. Hairless (IAF/HA-HO) guinea pigs immunized with Freund’s complete adjuvant containing Mycobacterium tuberculosis and repeatedly skin tested with purified protein derivative of tuberculin (PPD) display high levels of stable delayed-type hypersensitivity (DTH) to PPD. Met-enkephalin (YGGFM) and two of its metabolites (YGG, YG) enhanced and accelerated PPD-elicited DTH inflammatory reactions when injected together with elicitor in these animals. At 24 h, 5 × 10<sup>–3</sup> pmol met-enkephalin significantly enhanced DTH responses by 30% over PPD alone, while 5 × 10<sup>–5</sup> pmol of YGG and 5 × 10<sup>–9</sup> pmol of YG significantly enhanced these responses by 62 and 32%, respectively. At much higher doses (5 × 10<sup>3</sup> pmol), met-enkephalin and its metabolites significantly suppressed DTH reactions by 25–32%. Tyrosine and glycine had no effect on PPD-elicited DTH. All DTH reactions (control, enhanced, suppressed) displayed typical perivascular mononuclear cell infiltrates. We conclude that the immunoactivity of met-enkephalin resides in its first two amino acids and suggest that cleavage of enkephalin molecules to YG occurs in serum and/or on the cell surface.