Skimmed milk revealed to be the most efficient of the assayed blockers, as also observed by other authors. In the case of pork, this would be the only suitable blocking agent but avoiding the highest assayed muscle crude extract concentration. All the same attention must be paid, since some authors pointed out the possibility that skimmed milk may interfere the antigen-antibody reaction under certain conditions Vogt et al., 1987. For ELISA quantification of cathepsin L from bovine species, goat serum and BSA, together with skimmed milk, could be used as effective blockers always that a surcoating steps be included in addition to be present in the assay buffer.Marie Curie Individual Fellowship (Q1-CT-2002-51527)