Non-specific reaction has been a problem in doing, especially, research and diagnosis for infectious agents. Avidin-biotin-peroxidase complex (ABC) techniques has widely been used to amplify a reaction. Photobacterium damse1a subsp. piscicdia (formerly Pasteurella piscicida) exhibited a capacity to bind with streptavidin non-specifically. The band, estimated 26 K Da in Western blotted paper, was blocked with biotin but incompletely. In an attempt to explore an involvement of the non-specific substance in attaching piscine cells, cell attachment test performed using anti- Ph. d. subsp piscicida sera raised mouse and rabbit exhibited slightly blocking effects for Mediterranean (1736) and significantly for Japanese (Sp 92144) isolate. Biotin decreased the attachment ability significantly for Sp92144 but it was not effective to 1736. Both isolates showed greatly enhanced attachment ability with poly-L-lysin. The non-specific binding substance was contained in bacterial extracellular products (ECPs). The substance was able to purified with 2-imminobiotin affinity column, the purified substance appeared to have 4 bands in silver staining, and had a carbohydrate branch. This purified substance showed cytotoxic effects selectively between 5 piscine cell lines. Moreover, it stimulated rainbow trout macrophage in terms of reduction of cytochrome cas well as yeast phagocytosis, significantly.