Specificity: CA19-9, a carbohydrate epitope expressed on a high MW (>400kDa) mucin glycoprotein, is a sialyl Lewis a structure which is synthesized from type 1 blood group precursor chains and is present in individuals expressing the Lewis a and/or Lewis b blood group antigens. Immunogen: Precipitin lines obtained after immuno-diffusion using monoclonal antibody 116-NS-19-9 and mucins isolated from an ovarian cyst of a Lewis A+B- patient (0Le).