A series of experiments was conducted to establish a species identification method by means of the enzyme-linked immunosorbent assay (ELISA) using monoclonal antibody with high specificity, the result of which is reported. The limit of human IgG detection by inhibition ELISA using mouse anti-human immunoglobulin kappa light chain (Ig kappa) monoclonal antibody is 0.01 microgram. Using this method it was possible to detect human Ig kappa from bloodstains in dilutions up to 1:100,000 (dilution multiple of bloodstain extract to the original amount of blood). Human Ig kappa was detectable in human bloodstains in dilutions up to 1:50,000 one year after stain preparation and in dilutions up to 1:10,000 4 years after stain preparation, showing that this method is applicable to species identification of old bloodstains. When primates other than human were studied for specificity, red-chested tamarin and Japanese monkey showed no cross reaction at all. Cross reaction was noted with a chimpanzee, but experimentally it could be differentiated from human. Furthermore, no cross reaction was noted with respect to blood samples from non-primates (guinea pig, sheep, horse, dog and cat).