The inhibitory effects of clove, cinnamon, cardamom, turmeric and pepper on the histamine production and histidine decarboxylase activity of Morganella morganii (a potent histamine-producing bacteria in fish) was examined at 30 degrees C using HPLC. Cinnamon and clove exhibited a significant (P < 0.01) inhibitory effect, whereas turmeric and cardamom had a moderate effect. These spices were applied to whole mackerel at a level of 3% and their inhibitory effect on biogenic amine formation at 30 degrees C was also examined. As in the in vitro study, clove and cinnamon showed a significant (P < 0.05) inhibitory effect on histamine, putrescine and tyramine formation but not on that of cadaverine. Cardamom and turmeric exhibited a moderate effect and pepper was ineffective. Therefore, clove and cinnamon are more helpful than cardamom and turmeric in the minimization of the formation of toxic histamine in mackerel.