Citrinin, a relatively weak mycotoxin, can be detoxified by heating; however, a compound having high toxicity was formed on heating at 140°C in the presence of water. The toxic compound was isolated from heated citrinin and its structure was determined. Its toxicity, evaluated by cytotoxicity assay, was 10-fold higher on a weight basis than that of citrinin. This new compound was named citrinin H1, which was also formed by heating citrinin at 100°C for 30 min.