Groups of male and female rats and mice were exposed to 0.05 and 0.15 ppm of toluene-diisocyanate (TDI) by inhalation for 6 h/day, 5 days/week for approx. 2 years. Type and incidence of tumours and the number of tumour-bearing animals of either species did not indicate any carcinogenic effect. Haematology, biochemistry, urinalysis, and cytogenicity did not reveal any untoward effect. Increased mortality (females only), reduced weight gain and signs of irritation in the upper and lower respiratory tract resulted from exposure to TDI in the mouse study with the highest incidence in the 0.15 ppm exposure level. Male and female rats of the 0.15 ppm group gained less weight during the first 12 weeks of the study. Histopathological examination in the rat study has not been completed, but no effect in the respiratory tract or in any other tissue has yet been seen.