The subchronic toxic effects of thiabendazole (TBZ) administered in the diet at levels of 0 (control), 0.8 and 1.6% for 13 wk to male and female ICR mice were investigated. Mean body weights of male mice fed 0.8 or 1.6% TBZ showed a significant decrease compared with controls, except for wk 3 and 8 for mice fed 0.8% TBZ. Red blood cell parameters in male mice of treated groups were significantly lower than controls. Biochemistry showed increased concentrations of GOT and GPT in male and female mice of the 1.6% TBZ groups. Relative spleen or liver weights were significantly increased in male and female mice of treated groups. Relative kidney weights of treated mice tended to be increased in comparison with controls. Histological findings showed a marked haemosiderosis and extramedullary haematopoiesis in the spleen of treated mice. In the liver, sinusoidal dilatation and enlargement of liver cells were found in treated mice. In the kidney, atrophy of tubules with peritubular fibrosis, cell infiltration and some tubular necrosis were found in treated mice. Slight hyperplasia was found in the urinary bladder of treated mice. The findings in the present study indicate that TBZ caused a slight anaemia and liver or kidney injury at both levels tested, under these conditions.