Abstract The observation that various aromatic amine derivatives were inhibitory in classic immune lytic systems prompted us to investigate the effect of tryptophan metabolites on in vitro lysis of PNH erythrocytes. PNH erythrocytes and GSH-treated normal erythrocytes (previously shown to assume PNH-like lytic features) were incubated in acid-serum lysis and sucrose lysis studies with and without addition of various aromatic amine derivatives. Addition of 0.03M. acetyl-1-tryptophan resulted in a 70 to 80 per cent inhibition of acid-serum lysis of PNH cells. A direct relationship between the metabolite concentration and the per cent inhibition was characteristic of all studies. A hierarchy of inhibitory potential was demonstrated ranging from 100 per cent to zero. Nearly identical results were obtained with natural PNH and GSH-treated erythrocytes. Similar observations were made with respect to sucrose lysis of PNH erythrocytes. These studies demonstrate inhibition of in vitro PNH erythrocyte lysis by tryptophan metabolites. They illustrate another similarity in lytic behavior between natural PNH cells and GSH-treated normal erythrocytes.