The current threshold limit value (TLV) of 100 ppm for methyl methacrylate vapor was originally set because it was considered to be a value sufficiently low to preclude either systemic toxicological effects or discomfort from irritation in workers chronically exposed to the vapor. Shortly after this limit was proposed we became interested in evaluating the toxicological properties of methyl methacrylate vapor and, subsequently, of providing experimental data which would be useful in evaluating the adequacy of this TLV. The purpose of this article is to review some of the more relevant literature prior to 1976 which pertains to inhalation exposure and to summarize our more recent data which resulted from statistically designed studies employing acute and subchronic exposures to known concentrations of the vapor including the near-TLV value of 116 ppm.