A summary of data from published and unpublished literature indicates methyl methacrylate (MMA) did not show evidence of mutagenicity in the Ames Salmonella/microsome assay, the BHK21 mammalian cell transformation assay, nor in the chromosomal effect study with mice. No evidence of chromosomal damage was seen in a test with bone marrow cells of rats. Rats exposed to MMA vapor up to 400 ppm for 2 years and hamsters exposed for 18 months showed no observable toxic effects. Beagle dogs exposed similarly for 3 months and monitored especially for cardiovascular effects showed no evidence of adverse effects. No teratogenic effects were seen in rats exposed to up to 1000 ppm MMA vapor from gestation days 6 to 15. The acute oral LD50 for rats is 7.5 to 15 g/kg, for mice, 5.9 g/kg, and for guinea pigs, 6 g/kg. The acute dermal LD50 for rabbits is 7.5 g/kg. Data suggest MMA is a dermal sensitizer to guinea pigs and possibly to humans, and that it is irritating to rabbit eyes. Pharmacokinetic studies with rats indicate MMA is rapidly and extensively degraded to carbon dioxide and eliminated via lungs, irrespective of route of administration. A survey by NIOSH of 91 exposed and 43 non-exposed workers indicates there were no marked effects on health from exposureto MMA. The 96-hour LD50 values for freshwater fish range from 159 to 368 g/cubic meter, and concentrations of 37 mg/L and higher inhibits cell multiplication in aquatic microorganisms.