Twenty-three protozoan-free lambs were experimentally infected with Giardia duodenalis trophozoites at 6 weeks of age, while 24 controls lambs were not challenged. Weekly blood samples were taken and faecal cyst counts monitored for 11 weeks following infection. All experimentally infected lambs remained infected throughout the 11 week study period and control animals remained free of the parasite. Giardia-specific serum IgM, IgG and IgA antibody titers were determined weekly in 10 infected and 10 control lambs by enzyme-linked-immunosorbent assay (ELISA). Western blot analysis of serum immunoglobulins to proteins derived from four different Giardia isolates (S2, WB, D3 and NF) was performed. Weekly mean control IgM, IgG and IgA titers did not change throughout the study. Infected lambs showed no difference in IgM titers between weeks 0 and 11, but IgG and IgA titers of infected lambs differed from the preimmune (week 0) serum titer at weeks 5, 7, 9, 11 and weeks 5, 9 and 11 respectively. SDS polyacrylamide gel electrophoresis analysis showed homogeneity in the proteins of the four Giardia isolates. Antigenic proteins were also similar for all four isolates; however, the proteins recognized by IgM, IgG and IgA antibodies differed. The weak immune response of lambs to Giardia may account for the chronic nature of this disease in sheep.