When certain bovine sera (BS) were used to culture P3HR-1 cells, the amount of infectious Epstein-Barr virus (EBy) released into the culture fluid was significantly less than that obtained when calf serum (CS) or fetal calf serum (FCS) was used. A direct dose-response relationship was noted between the concentration of BS and the degree of inhibition of virus production. Cell growth was not inhibited nor was virus Infectivity neutralized by BS. Similarly, EBV infectivity was not reduced by spent medium containing BS. The inhibition of virus production by BS could be reversed by replacement of the serum supplement with CS. The formation of viral capsid antigen and of infectious virus within the cells was not inhibited by BS, whereas the amount of virus released was significantly decreased in comparison with culture media containing FCS or CS. These findings suggested the presence of a lactor(s) in the BS which inhibited the release of EBV from the cell.