Microcalorimetric bioassay for acute cellular toxicity is based on metabolic heat production from cultured cells. The biological response to toxicants is the inhibition of the heat production rate in cells, and toxicity is expressed as the concentration of toxicant that is 50% effective in this inhibition ( IC 50 ). In this paper, the effect of Na 2SeO 3 on Bacillus subtilis growth was investigated at 37 ℃ by microcalorimetry. The relationship between growth rate constants ( k ) and concentration of Na 2SeO 3 ( c ) shows a logarithmic normal distribution, and IC 50 is 20.3 μg/mL. All these thermokinetic information is readily obtained by an LKB 2277 204 heat conduction microcalorimeter. Microcalorimetry is a quantitative, inexpensive, and versatile method for toxicology research. Microcalorimetric bioassay for acute cellular toxicity is based on metabolic heat production from cultured cells. The biological response to toxicants is the inhibition of the heat production rate in cells, and toxicity is expressed as the concentration of toxicant that is 50% effective in this inhibition ( IC 50). In this paper, the effect of Na 2 SeO 3 on Bacillus subtilis growth was investigated at 37 ° C. by microcalorimetry. The relationship between growth rate constants (k) and concentration of Na 2 SeO 3 (c) shows a logarithmic normal distribution, and IC 50 is 20.3 μg / mL. All these thermokinetic information is to be obtained by an LKB 2277 204 heat conduction microcalorimeter. Microcalorimetry is a quantitative, inexpensive, and versatile method for toxicology research.