SUMMARY In an attempt to correlate host and parasite-related events occurring during the course of a primary Giardia infection in the mouse we have measured epithelial cell kinetics, enzymes, and intraepithelial lymphocytes at different stages of the infection. New methods were developed for the accurate measurement of parasite numbers and distribution within the gut. In jejunum a modest decrease in villus length and intraepithelial lymphocytes at week 1 preceded a pronounced disaccharidase deficiency at week 2, the time of maximum trophozoite numbers, whereas crypt lengthening and increased cell production became maximal at week 3. As trophozoite numbers fell the intraepithelial lymphocyte count and disaccharidase values rose. With the exception of the intraepithelial lymphocyte count, which followed the same pattern as in jejunum but two weeks later, the changes seen in the ileum were the opposite of those in jejunum, suggesting rapid ileal adaptation. The results indicate that the disaccharidase deficiency associated with giardiasis is likely to represent a direct effect of the parasite on the brush border rather than enterocyte immaturity, whereas the intraepithelial lymphocyte response reflects host immunity to the parasite. Profound adaptive changes occur throughout the small intestine in response to a relatively localised insult.Infection with Giardia lamblia may be entirely asymptomatic, may produce a mild, self-limiting illness, or chronic diarrhoea with or without malabsorption. The reasons for this variation in severity are unknown at present, but among the possible causes to be considered are variations in parasite virulence, associated bacterial overgrowth in the small intestine, nutritional status, and other host factors such as the type of the immune response and its effect on the intestinal mucosa. Various abnormalities of small bowel pathology have been identified consistently in giardiasis, and these include disaccharidase deficiencies 12 increased intraepithelial lymphocyte counts3 4 and, in patients with malabsorption, crypt hyperplasia with short villi and increased lamina propria cellularity.5 6 The severity of the mucosal changes in a biopsy of proximal jejunum appears to correlate with the * On leave from the

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