A series of experiments was performed in vitro and in vivo to determine the influence of FK-565, a heptanoyl tripeptide, on lymphocyte and macrophage function in swine. Compared with values for control cultures, mitogen-stimulated lymphocyte blastogenesis and interleukin-2 production were unaffected in cells preincubated with 0.1, 1.0, and 10.0 micrograms of FK-565/ml. Natural killer cell activity was increased by preincubation with 1.0 microgram of FK-565/ml; however, this increase was not statistically significant. In vitro treatment of porcine alveolar macrophages with FK-565 did not enhance cytolytic activity or bactericidal activity. In in vivo experiments, FK-565 given orally to pigs at concentrations of 6 or 60 micrograms.kg-1.d-1 for 5 days did not affect lymphocyte blastogenesis, interleukin-2 production, or alveolar macrophage bactericidal activity. A trend toward increased natural killer cell activity was evident in pigs treated with FK-565. In contrast, pigs treated with 6 micrograms.kg-1.d-1 had significantly (P less than 0.01) decreased alveolar macrophage cytolytic activity. These data indicate that at the dosages tested, FK-565 is not a suitable immunomodulator for enhancement of nonspecific immunity in swine.