Initial cellular events in the intestinal immune response occur within Peyer's patches and are subject to complex regulation by T cells. The aim of this study was to analyze the response of murine Peyer's patch T, T-helper (Th), and T-suppressor (Ts) cells to Giardia muris infection. Immunocompetent BALB/c mice were infected with G. muris cysts and, at serial times during the infection, Peyer's patch leukocyte suspensions were incubated with fluorescent monoclonal antibodies that identified murine leukocytes, T, Th, or Ts lymphocytes. These suspensions were examined by flow cytometry to quantify each T-cell subset as a percentage of total leukocytes. Total Peyer's patch leukocytes more than doubled in number during the course of G. muris infection and returned to control levels as the infection was cleared. The percentages of Peyer's patch Th and Ts lymphocytes were 34.1 +/- 0.8% (mean +/- SEM) and 6.2 +/- 0.3%, respectively, in the absence of infection, and did not change significantly during Giardia infection. The Th/Ts ratio in Peyer's patches was 5.6 +/- 0.2 in uninfected BALB/c mice and also did not change significantly during clearance of G. muris. We conclude that Peyer's patch leukocytes double in number in response to G. muris infection in immunocompetent mice, G. muris infection does not lead to altered percentages of Peyer's patch T, Th, or Ts lymphocytes, and clearance of G. muris infection is associated with a Peyer's patch Th/Ts ratio of greater than 5.