5-Hydroxy-1-methylbenzimidazole and 5-methoxy-1-methylbenzimidazole are more active than 5-methyl-2-D-ribobenzimidazole both with respect to specific activity and maximal enhancement at the highest tolerated dose. The hydroxyl substituent is superior to the methoxyl grouping. Substitution at position 5 is superior to substitution at position 6 with respect to the tolerated dose level and therefore the maximal effect obtainable, but the 6-hydroxy-1-methyl derivative showed the highest specific activity.

5-Methoxy-1-methylbenzimidazole increases the yield to a comparable extent as measured by infectivity and hemagglutination titrations. The responses of membranes from individual chicken embryos to the enhancing action of 5-methoxy-1-methylbenzimidazole and 5-methyl-2-D-ribobenzimidazole are similar. 5-Methoxy-1-methylbenzimidazole restores the capacity of membranes from older chicken embryos to produce a large amount of virus after a small inoculum. This derivative increases the yield of virus in membranes treated before infection only. Maximal enhancement is obtained with prolonged treatment, starting before, and continuing after infection. 5-Methoxy-1-methyl-benzimidazole increases the yield of virus from COFAL-negative embryos in which the control yield is very low. Combined treatment with moderate doses of 5-methoxy-1-methylbenzimidazole and 5-methyl-2-D-ribobenzimidazole gives an additive effect.