We describe a radioimmunoassay for lysozyme (EC 3.2.1.17, mucopeptide N-acetylmuramohydrolase) in plasma and urine. It involves use of an antibody, raised in rabbits, to lysozyme from urine from a patient with monocytic leukemia. This antiserum, diluted 4000-fold, is incubated with radiolabeled lysozyme for 2 h and antibody-bound lysozyme is separated from free lysozyme with dextran-coated charcoal. Validation of the assay included precision and parallelism studies with serum and urine samples from patients with monocytic leukemia and analgesic nephropathy. Results of the radioimmunoassay and those obtained with a Micrococcus lysodeikticus lytic assay correlated well (r = 0.94). Sensitivity of the assay was 3.5 micrograms/L for both serum and urine. The main advantages of the assay are its good precision and reproducibility and its high sensitivity.