The influence of glucose concentration on Cd, Cu, Hg, and Zn toxicity to a Klebsiella sp. was studied by following the degradation of C-labeled glucose at pH 6.0. Uptake of C into the cells was also determined. The carbon concentrations ranged from 0.01 to 40 mg liter, which are equivalent to soluble C concentrations in natural environments. The toxicity of Cu, Cd, and Zn to a Klebsiella sp. was affected considerably by the C concentration. Copper at 10 M was toxic when the carbon concentration was 10 or 40 mg liter, while at 0.01 to 1.0 mg liter no toxicity was observed. Cadmium and zinc were toxic at 10 M in media containing 0.01 to 1.0 mg of C liter. At C concentrations greater than 1.0 mg liter, the inhibition of glucose degradation and carbon assimilation was observed at 10 M Cd and Zn. The toxicity of mercury seemed to be independent of the C concentration. Results of this study showed that the nutritional state of an organism may have a profound effect on its sensitivity to metals. Metals taken up by an energy-driven transport system may be less toxic under conditions of C starvation. The C concentration should be taken into account when evaluating results from toxicity studies, especially as most microorganisms in nature live under energy-limited conditions.