To determine the effects of protein malnutrition on the severity and duration of infection with Giardia lamblia, Mongolian gerbils were pair-fed a pelleted control (C) diet (20% protein) and a low-protein (.5%; LP) diet for 3 weeks before and after being infected with 100,(MM) cysts orallv. Weight loss, fecal fat, enteropooling. and the duration of cyst excretion were all greater in the infected LP than in the infected C animals. During peak infection the upper intestinal intraepithelial lymphocyte infiltration, crypt enlargement, and villus enterocyte migration were greater in C than in LP animals, as was the villus mast cell number at the end of the infection. It is concluded that in the protein-malnourished host the increased severity of Giardia infection correlates with a reduction in enterocyte production and migration, probably secondary to a reduced lymphocyte infiltration, and the increased infection duration correlates with blunted mast cell migration into affected villi.