An immunofluorescent test, using Giardia lamblia cysts as antigen, gave positive results in 32/36 cases of giardiasis with malabsorption, 0/2 cases of giardiasis without malabsorption, and 0/17 control patients without giardiasis or malabsorption. The test was positive in 10/34 patients with malabsorption in whom G. lamblia could not be detected by stool examination or biopsy; some of these cases were presumed to be cryptic giardiasis. There was a crude correlation between antibody titre and the severity of the histological lesion in the jejunum. The finding of a reliable source of antigen remains a problem.