An enzyme linked immunosorbent assay(ELISA)detected the presence of anti-koi herpesvirus(KHV)antibodies in the serum of koi or colored carp(Cyprinus carpio)following either natural or experimental exposures to KHV.Concentrations of anti-KHV antibodies were detected at serum dilutions as great as 1:62, 500 in a population of koi kept in virus-free water for 1 year following a naturally occurring outbreak due to KHV.At serum dilutions less than 1:2, 500 cross reactions with a second herpes-like viral agent Cyprinid herpesvirus 1 (CyHV-1) was detected in serum from both experimentally and naturally KHV exposed koi. Passive immunization by administration of anti-KHV antibodies from koi recovered from previous virus infections to naive koi provided only partial and transient protection to waterborne challenges with KHV. Koi that maintained high levels of serum anti-KHV antibodies after 1 year in virus-free water are deemed as suspect carriers of the virus. The identification of suspect carriers by screening of koi and common carp populations, including potential broodstocks, with the KHV ELISA should improve the ability to control this important viral pathogen.