A sensitive, reproducible enzyme-linked immunosorbent assay using unfixed, intact Giardia lamblia trophozoites as antigen is described. Employing this assay, we demonstrated that 81% of 59 symptomatic giardiasis patients and 12% of 17 uninfected control subjects had circulating IgG antibodies to G. lamblia. Eleven out of 15 patients tested serially had detectable antibody from 2 wk to 15 mo after therapy for acute infection. The prevalence of antibody in 197 unselected individuals in the Washington, D.C. area was 14%. Specificity of the antibodies was demonstrated by the ability of live G. lamblia trophozoites, but not Entamoeba histolytica or Trichomonas vaginalis trophozoites, to absorb the antibody activity. High titers of antibody were detected in 5 out of 7 patients with recurrent infection. The enzyme-linked immunosorbent assay described here may help to further characterize the antibody response to infection with G. lamblia. © 1981.