A staphylococcal coagglutination test was developed for the rapid detection of infectious hematopoietic necrosis virus (IHNV) in cell cultures and infected fish. The test could be completed in 15 min but required a minimum IHNV titer of 10(6) PFU/ml to obtain a positive reaction. All IHNV isolates, representing the five electropherotypes taken from a wide variety of species and different geographic ranges, caused coagglutination of Staphylococcus aureus cells sensitized with rabbit polyclonal serum against the Round Butte IHNV isolate. The coagglutination reaction was blocked by preincubation of IHNV with homologous antiserum, and IHNV did not cause coagglutination of S. aureus cells sensitized with normal rabbit serum. In specificity tests, cells sensitized with rabbit anti-IHNV serum or normal serum did not coagglutinate in the presence of infectious pancreatic necrosis virus, viral hemorrhagic septicemia virus, cell culture medium components, or media from cultures of cell lines of salmonid and nonsalmonid origin. Most importantly, the coagglutination test was able to detect and identify IHNV directly from experimentally infected rainbow trout fry, the organs of naturally infected adult kokanee salmon and winter steelhead trout, and ovarian fluids of the winter steelhead trout. The coagglutination test is very suitable for field use, since it is inexpensive, simple to interpret, sensitive, and rapid and requires no specialized equipment.