An enzyme-linked immunosorbent assay was devised in order to search for antibodies against Blastocystis hominis in infected humans. Reaction proteins were obtained from washed, axenic B. hominis cells, as sonicate. Sonicate was diluted to provide 17 and 34 micrograms of protein per well. Dilutions of patients' sera were applied, followed by phosphatase-conjugated goat anti-human serum and phosphatase substrate. Color was measured at 405 microns wavelength. Immunoglobulin G antibodies to high titers were found. Of 30 sera tested from 28 patients, 3 were negative at the 1/50 threshold dilution, 8 were positive at 1/50, 3 at 1/100, 2 at 1/200, 3 at 1/400, 6 at 1/800, and 5 at 1/1,600. Normal sera (42 blood bank sera) were all negative at 1/50. Each serum was subjected to multiple testing. Duplicate tests were included for each run, and runs were made from 4 to 6 for each serum. Blastocystis hominis is increasingly recognized to be a cause of human enteric disease, with symptoms often like those in giardiasis. Demonstration of strong antibody response is consistent with this view.