Recombinant human gamma-interferon was shown to inhibit the growth of Chlamydia trachomatis (L2/434/Bu) in HEp-2 cells. This inhibition could be reversed by the addition of tryptophan. The effect of tryptophan was dose dependent and determined by the interferon concentration. At low concentrations of interferon, the addition of tryptophan completely restored C. trachomatis infectivity, whereas at high concentrations (100-1000 IU/ml) the effect of interferon could not be totally reversed. The reversal effect of tryptophan could be achieved even when the addition was 48 h after infection. The probability that tryptophan degradation induced by gamma-interferon might be the mechanism involved in its antichlamydial activity is discussed.