Rats maintained for a period of 5 days on a diet containing purified lectins extracted from a Brazilian variety (called 'Jalo') of white kidney beans (Phaseolus vulgaris) developed marked ultrastructural changes in the epithelium of the proximal jejunum, while both pair-fed and ad-libitum-fed controls did not. The jejunal absorptive cells of rats fed a diet containing lectins exhibited conspicuous abnormalities of the microvilli. They were shorter, slightly thicker, irregular and more sparse; some were bi- or tri-furcated, sharing a common base of implantation. A slightly disorganized terminal web was present below the brush border. The supranuclear cytoplasm of a great number of cells exhibited large cytolysosomes. Comparison with the results of pair-feeding suggests that purified bean lectins have a direct causative role in the pathogenesis of absorptive cell changes in the jejunal villi of rats. The possible pathogenic mechanism of these lesions is discussed.