The aim of this work was to identify the T cell subset (helper/inducer or cytotoxic/suppressor) that plays a major part in the clearance of Giardia muris infection. BALB/c mice were selectively depleted of helper/inducer (Th/i) or cytotoxic/suppressor (Tc/s) T cells, by treatment with rat IgG monoclonal antibodies directed against the Th/i cell surface antigen L3T4, or against the Tc/s cell surface antigen Ly-2, and were infected with G. muris cysts. While mice depleted of Tc/s cells cleared G. muris infection at a normal rate, mice depleted of Th/i cells did not clear the infection. Depletion of the relevant T cell subset was confirmed by flow cytometry and by immunohistochemistry. The data indicate that helper/inducer T cells, but not cytotoxic T cells, are of major importance for elimination of G. muris from the mouse intestine.