Although it is widely accepted that Whipple's disease is caused by microorganisms, there is little agreement as to the exact nature of those microbes. Several different types of bacteria have been reported from patients with Whipple's disease causing some workers to speculate that Whipple's disease may be due to a variety of microorganisms. Using an indirect immunofluorescence technique, we have demonstrated bacterial antigens in the granules of the foamy macrophages from three patients with Whipple's disease. The macrophages in each case stained with several types of antibacterial grouping sera. Jejunal biopsies from three normal subjects and one from a patient with celiac-like disease did not show significant reactions with any of the antisera. When these same antisera were reacted with homologous and heterologous bacteria, numerous cross-reactions were seen. However, each organism only reacted with certain antisera resulting in a distinctive pattern of reaction which could identify it. Therefore, if a different organism was present in each case of Whipple's disease, the antisera should give dissimilar reactions from case to case, but if the same type of microbe was present in each case, a similar pattern of reaction should be seen. In fact, the reactions were remarkably similar from case to case. These results suggest that a single, antigenically definable microorganism is present in the jejunal tissue of patients with Whipple's disease.