Sin Nombre hantavirus (SNV) is the primary etiologic agent of hantavirus cardiopulmonary syndrome (HCPS) in the United States and Canada. Hantavirus cardiopulmonary syndrome is a zoonotic disease. The most common reservoir is the deer mouse (Peromyscus maniculatus), although numerous other species of wild rodent can carry the viruses that cause HCPS throughout the Americas. Infected rodents show no signs of clinical disease but they develop persistent infection. Sin Nombre virus can be contracted by exposure to feces, urine, or saliva of its rodent reservoirs. Detection of infection in rodents is most often based upon detection of specific antibodies; many laboratories use enzyme linked immunosorbent assays (ELISAs), which require a specialized electrical ELISA reader. Enzyme linked immunosorbent assay readers are not readily amenable to field usage. We describe a portable test, the strip immunoblot assay (SIA), which we have utilized in field diagnosis. The test can be conducted in approximately 6 hr during the day or can be conducted overnight. The test can be used to detect rodents positive for SNV antibody while they are in traps. We show that results with the SIA have excellent concordance with western blot and reverse transcriptase polymerase chain reaction tests.