The microcalorimetric bioassay for acute cellular toxicity is based on metabolic heat production from cultured cells. Microcalorimetry is a quantitative, inexpensive, and versatile method for toxicology research. 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 (IC50). In this paper, the effect of Cd2+on Rhizopus nigricans growth was investigated at 25℃ The relationship between growth rate constants (k) and concentration of Cd (C) shows a logarithmic normal distribution, and described as k=1.2742×1061 exp[-1.810×10-3 (C+2 283.0)2], and IC50 is 0.72μg/ml. These signals are readily obtained by an LKB 2277-204 heat 50 conduction microcalorimeter.