Human peripheral blood mononuclear cells, monocytes-macrophages and T-cells were stimulated with human recombinant interferon-gamma, interferon-alpha and phytohemagglutinin. The culture supernatants were analyzed for tryptophan, kynurenine, 3-hydroxyanthranilic acid, anthranilic acid and neopterin by high performance liquid chromatography. Tryptophan was decreased and the four other compounds were increased in supernatants of peripheral blood mononuclear cells activated by interferon-gamma (250 U/ml), interferon-alpha (10.000 U/ml) and phytohemagglutinin (1 microgram/ml). After splitting of peripheral blood mononuclear cells by adherence, the monocytes and macrophages but not the T-cells degraded tryptophan upon stimulation by interferon-gamma in a dose dependent manner. Supernatants of phytohemagglutinin stimulated but not of resting T-cells were found to induce tryptophan degradation by macrophages, the active principle being neutralized by an antiserum for interferon-gamma. Thus phytohemagglutinin acts by activating T-cells to release interferon-gamma which in turn induces macrophages to degrade tryptophan. In all experiments the appearance of neopterin in the culture media was correlated to the observed tryptophan degradation.