Arylsulfatase A and B activities have been determined in urine samples from four patients who received a Kidney allograft from living donors as a treatment for terminal uraemia. The values have been compared with those obtained in the urine of the respective donors. Two patients, with optimal renal functionality, the enzymatic activities were in the order of that observed in the urine of the donor in one case and lower than that of the donor in the other case. A patient showing kidney rejection episodes, reversed by the specific therapy, had the enzymatic activities higher than those shown by the donor. Another patient who suffered for two rejections, the last one irreversible, showed a constant higher value of the two enzymatic activities compared with those of the donor and a further increase during the rejection period. In the light of these preliminary results it seems that the determination of the arylsulfatase activities in the urine of transplanted subjects could contribute to establishing the functional activity of the transplanted kidney and also in establishing the tendency of the kidney to undergo rejection.