Following Giardia lamblia infection in immune intact NMRI mice, increased numbers of IgM-containing cells and decreased numbers of IgA containing cells were noticed in the lamina propria during the establishment (3-5 days) and acute (9-11 days) phases of infection. The decline in IgM-containing cells during the clearance phase of infection (17-21 days post-infection) was accompanied by an increase in IgA and IgG-containing cells. Our data suggest that the locally synthesized antibodies, especially of the IgA class, play a significant immunodulatory role in the clearance of G. lamblia infection from the gut. Mice immunosuppressed using rabbit anti-mouse lymphocyte serum or dexamethasone had significantly reduced numbers of IgA and IgG-containing cells during all phases of infection and higher parasite loads in their jejunum. It appears that one of the reasons for increased severity and chronicity of G. lamblia infection is the decrease in immunoglobulin-containing cells in the gut.