The principal aims of this work were (i) to identify the molecular weight (MW) of Giardia muris trophozoite antigens that are recognized by IgA in small intestinal secretions from G. muris-infected mice, and (ii) to determine whether mouse intestinal Giardia-specific IgA is directed against trophozoite surfaces. BALB/c mice were infected with G. muris cysts, and intestinal secretions were harvested from these mice at various times after the start of Giardia infection, and from uninfected mice. Flow cytometry showed that intestinal IgA from G. muris-infected mice, but not from uninfected mice, became bound to trophozoite surfaces in vitro. Western blotting of trophozoite proteins with mouse intestinal secretions showed that IgA from Giardia-infected mice reacted specifically with a broad protein band of approximately 30,000 MW. This finding suggests that one or more trophozoite proteins of approximately 30,000 MW are targets for intestinal antibody in mice infected with G. muris.