Through systematic investigation of several variables the agar plate assay for lysozyme was modified so that concentrations as low as 5μg egg white lysozyme per 100 ml of sample could be determined in 3 h. Plates were stained with 0.5 g buffalo black in 100 ml 7% acetic acid followed by destaining in 7% acetic acid until satisfactory contrast between clear zones of lysis and surrounding seeded agarose was obtained. Standard egg white lysozyme solutions remained stable for at least 7 days at −5 C. The best lyso-plate method included 1.0 g agarose with 0.1 g NaC1 in 100 ml of a pH 7.0 phosphate buffer seeded with 0.020 g Micrococcus lysodeikticus and incubation at 47 C for 3 h.