Lysozyme turnover studies with (125)I-labeled human lysozyme were carried out on 22 patients, viz. nine control patients, seven nephrological patients with varying degrees of renal insufficiency, including three bilaterally nephrectomized patients, and six hematological patients with disturbed turnover of the neutrophilic granulocytes. It was found that plasma lysozyme has a rapid turnover with a fractional catabolic rate of 76%/hr of the plasma content. Lysozyme catabolism varied with the endogenous creatinine clearance; in addition however, extrarenal sites of catabolism were demonstrated since lysozyme could be broken down in the anephric patients, although only at a rate amounting to about 15% of the rate found in persons with intact kidneys. In the uremic patients the increased plasma lysozyme concentration was due to decreased rates of catabolism; in the hematological patients the increased plasma lysozyme level was due to increased rates of synthesis which supports the hypothesis that plasma lysozyme mainly stems from disintegrating neutrophilic granulocytes. Furthermore, it was shown that in the nonhematological patients examined, the rate of synthesis varied with the endogenous creatinine clearance.