Human recombinant gamma interferon (rHuIFN-gamma) was found to induce tryptophan degradation in vitro in human cell cultures and in vivo in participants in phase I clinical trials. When human lung fibroblasts were treated with various concentrations of rHuIFN-gamma, they degraded tryptophan in a dose- and time-dependent manner. No tryptophan degradation was observed when cells were incubated in growth medium alone or in medium supplemented with human recombinant beta-interferon (rHuIFN-beta ser). Similarly human bladder carcinoma cells were induced to catabolize tryptophan after incubation with rHuIFN-gamma, but no activity was observed in untreated cells or cells treated with either rHuIFN-beta ser or human naturally produced alpha-interferon (HuIFN-alpha). When tryptophan plasma levels were measured in cancer patients who had received i.v. bolus injections of rHuIFN-gamma as part of a phase I clinical trial, decreased tryptophan levels were observed when compared with pretreatment values or values obtained from individuals who had received i.v. injections of HuIFN-alpha. Urine analyses were suggestive that plasma tryptophan degradation occurred via the kynurenine catabolic pathway in individuals who received rHuIFN-gamma. We conclude that tryptophan degradation is an activity induced in vitro and in vivo in response to exogenous IFN-gamma but not to IFN-alpha or IFN-beta. Tryptophan degradation may play an important role in the mechanism of antiproliferative, immunologic, and clinical side effects of IFN-gamma.