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 measuring acute toxicity. 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 Pb2+ on Rhizopus nigricans growth was investigated at 25 degrees C. The relationship between growth rate constants (k) and concentration of Pb2+ (C) is k = 0.04226 exp[-3.636 x 10(-5) (C + 177.0)(2)], and IC50 is 47.5 mu g ml(-1). These signals are readily obtained by an LKB 2277-204 heat conduction microcalorimeter. This system can therefore measure acute toxicity for a broad range of toxicants having target sites in various cellular metabolic pathways.