Previous studies have suggested that Giardia infections are cleared immunologically, but have not defined the mechanism of clearance. The aim of the present work was to compare subpopulations of leukocytes harvested from the intestinal lumen of immunocompetent BALB/c mice, which clear Giardia muris infection rapidly, with those of immunodeficient nude mice, which become chronically infected with Giardia muris. Leukocytes were obtained from the intestinal lumen of Giardia-infected mice, and subpopulations of these cells were quantified after immunofluorescent staining with monoclonal antibodies. Identical numbers of leukocytes were harvested from the intestinal lumen of Giardia-infected immunocompetent mice and nude mice, but the number of these leukocytes bearing the T-lymphocyte antigen Thy-1.2 was smaller in nude mice than in immunocompetent mice. In contrast, no striking differences were observed between the numbers of luminal cytotoxic/suppressor T lymphocytes or macrophages in Giardia-infected nude versus immunocompetent mice. The findings suggest that clearance of Giardia muris infection is not mediated by cytotoxic T lymphocytes or macrophages. Subsequently, T lymphocytes and T-lymphocyte subsets were quantified in cell suspensions prepared from Peyer's patches of immunocompetent BALB/c mice and nude mice. It was found that nude mice have a profound deficiency of Peyer's patch helper/inducer T lymphocytes. This deficiency may account for the inability of nude mice to clear Giardia muris infection at a normal rate.