An enzyme-linked immunosorbent assay for pepsinogen in sheep plasma is described. The assay requires only 20 microliter of plasma. As little as 250 ng pepsinogen/ml plasma can be measured using antibody-coated polystyrene tubes and if "MicroElisa" plates are used 50 ng pepsinogen/ml plasma can be assayed. The method is more specific, more sensitive, uses less samples, has lower blank values than the present widely-used spectrophotometric method and could easily be automated.