1. Jejunal biopsy specimens were obtained from seven patients with post-infective tropical malabsorption, four of whom were infected with Giardia lamblia.
2. Enzymic analysis of the biopsy homogenates revealed a selective reduction of certain enzymes (alkaline phosphatase, γ-glutamyl transferase) localized at least in part to the brush border. Other enzymes (leucyl-2-naphthylamidase, α-glucosidase) were unaffected.
3. Density gradient centrifugation confirmed the selective decrease in the brush-border activity but the equilibrium density of the organelle was normal. Homogenization studies revealed that the brush border was unusually fragile.
4. Apart from reduced activities of the basal—lateral marker enzyme 5′-nucleotidase and enhanced lysosomal fragility, other organelles showed normal properties. There were no significant differences in any of these parameters in the patients with or without giardiasis.
5. The patients were restudied after their jejunal morphology and clinical features had returned to normal. All these parameters, apart from a persistently reduced activity of 5′-nucleotidase, returned to normal.
6. It is suggested that the principal lesion in patients with post-infective tropical malabsorption affects the cell membrane, specifically affecting the absorptive—digestive capacity of the enterocyte. These findings suggest that there is direct brush-border damage rather than mucosal invasion by bacteria, viruses or Giardia lamblia.