The behaviour of serum and urinary lysozyme was investigated before and after renal transplantation in 20 patients. The mean postoperative observation time was 67.8 (10 to 212) days. In 11 patients with reversible olig-anuria due to prolonged preoperative ischaemia, lysozymuria lasted for a period of 17 days after surgery, whereas in 8 patients with immediate transplant function lysozymuria disappeared 7 days after transplantation. Serum lysozyme concentrations were markedly elevated before transplantation in all patients. In patients with transplant failure due to ischaemia, normalization of serum lysozyme levels was achieved 28 days after surgery; patients with immediate function showed normal serum lysozyme levels already 7 days after transplantation. Prolonged lysozymuria was also noticed in 2 cases with irreversible ischaemic transplant failure, in 1 case with recurrence of glomerulonephritis and in 1 further case with acute pyelonephritis in the transplant. In 7 cases with an acute renal rejection crisis, lysozymuria was evident 0.7 days before clinical diagnosis of rejection. Serum lysozyme levels showed a strong correlation with serum correlation with serum creatinine concentrations. Therefore, lysozymuria in renal transplant patients indicates tubular transplant damage of varied aetiology. Elevated serum lysozyme levels, on the other hand, seem to reflect a reduced glomerular filtration rate.