An electron microscopic and cytochemical study of the Whipple bacillus in jejunal biopsies from three untreated patients was made using fixation procedures developed for the satisfactory preservation of bacterial ultrastructure. The envelopes of the normal-looking bacilli present free in the lamina propria consisted of the following layers. (i) A cytoplasmic membrane with a triple-layered profile and a mean thickness (peak-to-peak distance) of 6.08 nm. (ii) A thick (20 nm) cell wall containing peptidoglycan; the wall had a hitherto undescribed inner layer that contained polysaccharides, possibly teichoic acids. (iii) Surrounding the cell wall, a surface membrane with a symmetric profile and a mean peak-to-peak distance of 4.74 nm. The ultrastructural pattern of the Whipple bacillus wall corresponds to that of Gram-positive bacteria, but with an additional surface membrane. This membrane is different from the outer membrane of Gram-negative bacteria because it has a symmetric profile, is thinner and has no periodic acid-Schiff (PAS)-positive components. Normal-looking bacilli were seen very rarely inside jejunal macrophages, but degenerating bacteria were abundant in these phagocytes. Electron microscopy and ultrastructural cytochemistry of Whipple bacilli inside jejunal macrophages of the three untreated patients showed that the degenerative process is a sequence that leads to the loss of bacillary forms and to the accumulation of bacterial remnants resistant to degradation by the macrophage. These remnants correspond to the innermost, polysaccharide-containing portion of the bacillus wall. The progressive accumulation of these PAS-positive wall remnants is the origin of the intramacrophagic inclusions that are important in the histological diagnosis of Whipple's disease. The reported results indicate that in the three patients studied, the Whipple bacillus multiplies extracellularly, the bacteria that are phagocytosed by macrophages being degraded.