SUMMARY

A plaque assay is described in which cytolytic activity of lymphocytic horio-meningitis virus (LCMV) is observed at low but not at high concentrations of infecting virus. Quantitation of the interference at high virus concentrations is detailed. Use of this assay during the course of the LCMV infection in L cells has shown that an interfering component is produced in abundance after the initial peak in infectivity in the medium has been reached. Thereafter the ability of the virus stock to form infective centres declines while the interference activity rises. Co-infection of L cells with high concentrations of one LCMV strain and low concentrations of any other tested strain prevents cellular destruction. When small virus inocula are used interference an be observed within single ' bull's eye ' plaques which exhibit concentric rings of lysed and intact cells. Simultaneous infection of L cells with an auto-interfering concentration f LCMV does not interfere significantly with plaque formation by mengo, vaccinia, and vesicular stomatitis viruses, but reduces four- to fivefold the number &plaques formed by the Amapari and Parana viruses related to LCVM. Interferon does not play a role in the system described. The physical and the immunological data on the interfering substance are consistent with the hypothesis that defective-interfering virus particles are present in LCMV tissue culture stocks