A calorimetric assay utilizing mitochondrial enzymes to cleave a tetrazolium salt (MTT) was used to quantitate the cytotoxicity of methyl parathion (MP), permethrin (PER), and limonene (LIM) on a established Heliothis virescens cell line. The results, which were rapidly obtained with a multiwell scanning spectrophotometer (96 well ELISA reader), correlated to a high degree with other traditional but cumbersome methods of assessing cytotoxicity. The toxic doses calculated to kill 50 percent of Heliothis cells following a 20 hour exposure to MP, PER, or LIM were 2.39, 3.92, and 2.84 µg/well, respectively. The MTT bioassay system described here can be broadly applied in the field of insect toxicology.