In this study, microcalorimetry and measurement of culture turbidity were applied to evaluate the As(III) toxic effect on the metabolic growth of Bacillus subtilis. Using a multichannel thermal activity monitor, the power-time curves of the metabolic activity of B. subtilis during growth in the absence and presence of various concentrations of As(III) were obtained and studied. The turbidity changes during B. subtilis growth with As(III) were investigated by ultraviolet-visible spectrophotometry and the data agree with the results obtained by microcalorimetry. As(III) of various concentrations has different effects on the metabolic growth of B. subtilis with biphasic dose-response relationships called hormesis [i.e., low-concentration stimulation (10 microg/mL) and high-concentration inhibition (20-160 microg/mL). Typical J-shapes of the relationship between the growth rate constant (k) and c, and toxicity at the half-inhibitory concentration (IC(50)) of 98.82 +/- 7.32 microg/mL were obtained. The similarity between the two methods corroborates the validity and sensitivity of the microcalorimetric technique to investigate the toxic effect of As(III) on microorganisms.