An enzyme-linked immunosorbent assay (ELISA) was developed for the detection of Giardia lamblia in human feces. The assay could detect between 37 and 375 trophozoites from cultures and between 12.5 and 125 cysts purified from human stool. Stool specimens were positive by ELISA in 36 (92%) of 39 patients with giardiasis; negative specimens came from patients with low numbers of parasites in their stool. In 10 patients stools became negative by ELISA after successful treatment. Stool specimens were positive by ELISA in three (2%) of 128 patients without demonstrable Giardia organisms in their stools; one positive specimen was from a child with IgA deficiency and diarrhea who responded clinically to metronidazole. The ELISA is a simple, sensitive, and specific diagnostic test for G lamblia that will be useful in diagnosis, in follow-up treatment, and in large-scale studies directed at defining the epidemiology and pathophysiology of G lamblia infection.