The influence of light intensity on the effects of 10-6 mol L-199Tc on growth, chlorophyll and carotenoid contents of bush bean plants was investigated. After germination and cotyledon excision, the plants were grown in a growth chamber either under low light (photosynthetic active radiation, PAR 144 μE m-2 s-1) or higher light (PAR 307 μE M-2 s-1) conditions. In plants grown under the higher light conditions, 99Tc hardly affected CO2-assimilation, dark respiration, pigment contents and growth. No toxicity symptoms were observed in these plants. Under low light conditions, 99T c significantly decreased growth and the concentration of chlorophylls. Toxicity symptoms in the form of chlorosis and necrosis developed. Transmission electron microscopy (TEM) observations revealed alterations of chloroplast ultrastructure comparable to those described for plants slightly affected by paraquat toxicity or by Mo-deficiency. Our results indicate that in the low light plants 99Tc induces damage in chloroplasts by peroxidation of membrane lipids. © 1991 Kluwer Academic Publishers.