The fluorescence activated cell sorter (FACS) was used for detecting circulating antibodies to the surface antigens on isolated colon epithelial cells (anti-colon antibodies) by indirect immunofluorescence. Anti-colon antibodies were found in the serum of 30 of 41 (73%) patients with ulcerative colitis. This incidence is much higher than one established in earlier reports by application of indirect immunofluorescence to colon tissue using the fluorescence microscope. The results suggest that FACS analysis is very useful for detecting antibodies to colon specific antigen.