This document provides methods and materials related to assessing organisms for the presence or absence of anti-virus antibodies. For example, this document provides methods and materials that can be used to determine whether or not an organism (e.g., a member of a swine species such as a pig) contains anti-PRRS virus antibodies. In other embodiments, this document provides methods and materials that can be used to determine if a particular organism received a vaccine version of a virus, was infected with a naturally-occurring version of the virus, or is naïve with respect to the virus.