Immuno-enhancing actions of carnosine, beta-alanine, homocarnosine, and gamma-aminobutyric acid were studied in ddY mice by evaluating plaque-forming cell reaction against sheep red blood cells. Animals were administered the test agents in prior to, or simultaneously with, various treatments that are known to reduce immune function such as administration of the anti-tumor agents, mitomycin C and 5-fluorouracil, immunosuppressant cyclophosphamide, antiinflammatory agent hydrocortisone, or cancer implantation and gamma-irradiation. Experiments were performed also in aged mice with reduced immune function. The administration of these drugs showed non-specific immuno-enhancing effects under all conditions examined and on all cell groups that may have been affected by these immunosuppressive stimulus.