Theophylline is said to produce an inhibiting effect on the replication of the Newcastle disease virus in chick embryo fibroblast cultures when used in conc. of 1 mM. When the cultures are treated with theophylline 24 hours prior to their infection or following one-hour adsorption the virus titer is lowered in the process of replication by about 2 lg as against the titer when the virus is replicated in theophylline-free cultures. The inhibiting effect consists in blocking or arresting the production of mature virions.