Gizzerosine [2-amino-9-(4-imidazoyl)-7-azanonanoic acid] was isolated from a fish meal which caused black vomit in broiler chicks. Those given a diet containing fish meal which caused black vomit or synthetic DL-gizzerosine secreted excess gastric juice. The excess juice resulted in gizzard erosion and black vomit. In all the experiments, amongst chicks that died, those without gizzard perforation were greater in number than those with such lesions. It is considered that gizzard perforation by excess gastric juice is not the most serious lethal effect of gizzerosine. Chicks have been found to exhibit wide variation in sensitivity to gizzerosine. In a 4-week feeding trial of a fish meal that caused black vomit, 25 out of 100 died but nine grew normally. It should therefore be possible to estimate the maximum permissible concentration of gizzerosine in practical broiler diets. In the case of manufacturing facilities in which vapour is used to dry the fish meal indirectly, the conditions which avoid gizzerosine formation are known. It is desirable to determine safe conditions for fish meal drying when the process uses hot air heated by a flame.