Methylenedi-para-phenylene ester of isocyanic acid was examined in Salmonella typhimurium tester strains TA1535, TA1537, TA1538, TA98 and TA100 with and without rat liver homogenate S9 fraction to provide metabolic activation. Using the plate incorporation technique at doses of 0.5, 1.0, 1.5, 2.0 and 2.5 mg/plate, the test article was found to be mutagenic to strains TA98 and TA100 in the presence of activation; the greatest effect (0.27 revertants/uMol) was observed in TA100. In the absence of activation, a mutagenic effect was not observed in any strains. The investigators reported that preliminary cytotoxicity screening had been performed.