1. Like 2- and 4-dimethylaminophenol, 4-aminophenol in the presence of oxyhaemoglobin forms numerous adducts with glutathione (GSH). Using 14C-4-aminophenol and 3H-glutathione, ten different thioethers were isolated, by h.p.l.c., with isotope ratios of 1:1, 1:2, 1:3, respectively. The structural identification of the different thioethers is under current investigation. 2. In erythrocytes of human and dog, and in dog blood, in vivo, the same pattern of 4-aminophenol conjugates with GSH was found. 3. In vivo, 5% of administered 4-aminophenol is converted into thioethers within erythrocytes, accompanied by a 60% decrease in the cellular GSH, indicating the role of erythrocytes in the biotransformation of xenobiotics.