Acute inhalation toxicity was evaluated in groups of 5 male and 5 female Wistar rats exposed to polymeric 4,4'-diphenylmethane diisocyanate at nominal concentrations of 384, 418, 500 or 523 mg/cubic meter and particle diameter of < 5 micrometers for 4 hours. The test atmosphere was generated from a stainless steel/glass air nebuliser. Mortality was observed in 1 animal in the 384 mg/cubic meter dose group, 4 in the 418 mg/cubic meter dose group, 6 in the 500 mg/cubic meter dose group, and 5 in the highest dose group. The LC50 value was calculated to be 490 mg/cubic meter (376-638 mg/cubic meter 95% confidence limits) by the probit analysis method of Finney. Clinical observations included closed eyes, labored respiration, mouth breathing and slightly pink fur on the back skin at the end of the exposure period. Gross necropsy revealed lung hemorrhage or edema; in the highest dose group, lungs were greyish and too wet.