Summary1. Cystine, lysine and arginine determinations have been carried out on urines from twenty‐eight cystinuric patients and 121 of their relatives. Urines from 250 unselected individuals have also been examined. The cystine was determined polarographically and the lysine and arginine microbiologically. The urines have also been examined for amino‐acids by two‐dimensional paper chromatography and by paper ionophoresis at pH 11‐5.2. There is a high positive correlation between the cystine and lysine contents over the whole range which extends in the case of cystine from 20 up to 800 mg/g. of creatinine and of lysine from less than 25 up to 1800 mg./g. of creatinine.3. The arginine content of the urine remains at normal (< 25 mg./g. creatinine) or near normal values until the cystine content reaches about 250 mg./g. creatinine and the lysine reaches about 550 mg./g. creatinine. Above these values large increases in arginine excretion up to 1800 mg./g. creatinine may be observed.4. It is suggested that there is normally a renal tubular resorptive mechanism which at one stage is common to and specific for cystine, lysine, arginine and ornithine, and that this mechanism is defective in cystinuria. When the process fails completely or nearly completely the four amino‐acids appear in the urine in large amounts. When it is only partially deranged, arginine and possibly ornithine are resorbed preferentially to cystine and lysine.The authors would like to thank Miss Barbara Warland and Miss Robin Worthington for considerable assistance with the family case work, and Mrs Ruth Jacobs and Mr B. Cohen for much help in the chemical determinations.Miss E. B. Robson was in receipt of a research grant from the Medical Research Council.The polarograph used in this work was purchased with a grant from the Central Research Fund of the University of London.