A plaque-reduction assay for the determination of neutralizing antibody to human coronavirus OC43 has been set up, using as a plaquing factor the fetal calf serum containing an OC43 inhibitor. A similar inhibitor has also been found in human sera both negative and positive for OC43 antibody. The inhibitor can be removed by treatment of sera with phospholipase C and is likely to be a lipoprotein of the high-density class. The new plaque-reduction assay appears to be a very sensitive test for both OC43 antibody determination and serodiagnosis of OC43 infections. It may also be very useful for studies of antigenic relationships between human and animal coronaviruses. However, before testing, sera must be routinely treated for removal of viral inhibitor.
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