Citrinin is one of the mycotoxins produced by Penicillium citrinum. We examined the decomposition products after heating citrinin in water at 140 degrees C and isolated a major product, citrinin H2 (3-(3,5-dihydroxy-2-methylphenyl)-2-formyloxy-butane). Citrinin H2 did not show significant cytotoxicity to HeLa cells up to a concentration of 200 microg/ml (% cytotoxicity: 39%) in 63 h of incubation, but citrinin showed severe toxicity at a concentration of 25 microg/ml (% cytotoxicity: 73%). HPLC analysis of citrinin after heating under various conditions indicates that citrinin H2 is mainly yielded from citrinin.