Conditions are described for the production of 0.3 to 0.7 NIH mouse reference standard units of interferon per cell from Ehrlich ascites tumour cells cultured as monolayers and induced by infection with Newcastle disease virus (NDV). Inclusion of theophylline (6 mm) in the medium increased the interferon yield three to four times. Cells infected with NDV started to lyse at about 15 h p.i., but infected, theophylline-treated cells lysed only 24 h p.i.
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