At present, the hemagglutination inhibition (HI) assay is the sole commercially available serologic method available to detect exposure to ophidian paramyxovirus (OPMV) in snakes. During 2006, 26 eastern massasaugas (Sistrurus catenatus catenatus) were collected, and blood was sampled to determine their OPMV status. Samples from each snake were divided into 3 aliquots and tested by using commercially available HI assays against the 4 OPMV isolates used in the 3 laboratories that offer the service. All snakes were tested for antibodies by using HI assays against the green tree python (GTP), San Lucan rattlesnake (SLR), and Aruba Island rattlesnake (AIR) isolates. Twenty-five snakes were tested for antibodies against the western diamondback rattlesnake (WDR) isolate. All samples tested against the GTP and SLR were positive (26/26), whereas 56% (14/25) of the WDR assays were positive, and none (0/ 26) of the AIR assays yielded a positive result. There was 100% agreement between the GTP and SLR assays, and complete disagreement between the SLR and AIR, as well as the GTP and AIR assays. Kappa statistics for the GTP– WDR, SLR–WDR, GTP–AIR, SLR–AIR, and WDR–AIR indicated that the assays had less than chance agreement. The results demonstrate that current HI assays are not reliable as a sole diagnostic assay in the eastern massasauga. Furthermore, HI assays need to be evaluated by using other parameters to determine OPMV exposure in eastern massasaugas.