Ultrafine platinum particles protected by poly (methyl acrylate-co-N-vinyl-2-pyrrolidone) were applied to the immunological detection of methamphetamine (MA). The polymer-protected ultrafine particles could chemically bind anti-methamphetamine monoclonal antibody to their surfaces. The antibody-fixed particles behaved like an antibody in the immunoreaction. The antibody-bound particles competitively reacted against the immobilized and the free antigens in the ELISA system. MA could be detected down to a concentration of ca. 10 ng/ml. In the immune-dot assay, 10-100 ng/ml of MA was detected within 30 min using the ultrafine particles.