Anti-Giardia antibodies were studied in 88 sera of patients with symptomatic and asymptomatic giardiasis by using the indirect immunofluorescent test and Giardia intestinalis trophozoites from an axenic culture as antigen. The sera of 40 healthy subjects were used as a control group and of 28 patients with other intestinal parasites (14 with Sarcocystis sp. and 14 with symptomatic Entamoeba histolytica infection) have been investigated for determination of test specificity. 82% of sera from patients with giardiasis presented G. intestinalis antibodies at titers of 1:16 to 1:256. Titers of at least 1:32, accepted as diagnostic and indicative of the infection, were found in 66% of patients with giardiasis. All of 18 patients with acute course of giardiasis had titers of at least 1:32. Only 5 sera (7%) from healthy control subjects or from patients infected with other intestinal parasites reacted at dilutions of 1: greater than or equal to 32. The indirect immunofluorescence test proves to be useful in detecting antibodies to G. intestinalis in the serum. However, the negative results occurring rather frequently in persons with chronic infection indicate a need for a more critical evaluation of serological tests in giardiasis.