The influence of cotyledon excision on the effects of 99Tc on Tc accumulation, growth and ultrastructure of bean seedlings was studied. In plants with or without cotyledons, 99Tc accumulated principally in leaves, although leaves of plants without cotyledons showed a significantly higher 99Tc concentration. Cotyledon excision substantially enhanced the toxic effects of 99Tc on growth. Light, scanning and transmission electron micrographs from 99Tc-treated plants without cotyledons display increased autophagic vacuole activity, accumulation of protein bodies in roots, and decreased starch content and severe ultrastructural alterations in chloroplasts. Our results support the hypothesis that 99Tc toxicity is mainly associated with anabolic processes in developing tissues. © 1990.