Results from a test, a 13-week subohronic inhalation (repeated dose) study (with mutagenicity assessment). The dose levels were 1,5 and 20 ppm acetic anhydride vapour; recovery groups were included to see whether any effects observed were reversible, Clinical observations of eye and respiratory tract irritation and reduced body weights were observed primarily at 20 ppm. Microscopic examination of tissues revealed signs of ictitation of minimal severity in the respiratory tract in most animals at the 5 ppm exposure level. At 20 ppm, all animals showed minimal to moderate signs of respiratory tract irritation. No effects were detected at 1 ppm. No mutagenic effects were noted. In animals exposed to the same levels (1,5 and 20 ppm) of acetic anhydride for 13 weeks and then allowed a 13-week period without exposure significant recovery from irritation effects was reported. In summary, the effects were consistent with those typical of substances which act as local/site-or-contact irritants. As a precautionary reaction to new toxicology infbrmation, the principal European suppliers of acetic anhydridel have decided to adopt a lower accupational exposure limit on a voluntary basis. They will adopt an occupation exposure limit of 1 ppm (averaged over 8 hour working day) and encourage other manufacturers and users of this material to meet this new lower limit.