Abstract
Summary: An improved method for the titration of antibodies against interferon has been described. The assay permits the use of 10 to 15 times less interferon for neutralization by antibody than conventional monolayer techniques. Neutralizing titers of anti-interferon sera are enhanced by a corresponding factor. Reading of the test is based on color change of the phenol red indicator in the medium, which can be recorded visually or in a photoelectric colorimeter. The assay has been used with L, HeLa and chick embryo fibroblast cells. Antibodies against L cell interferon developed in guinea pigs only after a prolonged course of immunization. Similar results were obtained in rabbits injected with L interferon and in guinea pigs immunized with chick interferon. HeLa interferon of low potency failed to elicit antibodies. Neutralization tests with anti-interferon sera disclosed that a) L interferon differs antigenically from the producer cell, b) L, chick and HeLa interferons posses different antigenic compositions and c) minor cross-protection of heterologous cells may be due to components in the preparations other than interferon.

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