Synthesis of viral-specific neuraminidase (NAase), one of the structural proteins of influenza virus, was inhibited by addition of 2-mercapto-1-(β-4-pyridethyl) benzimidazole (MPB), at a concentration of 50 μg/ml, to culture medium of infected HeLa cells during the first 1 hr post infection, and this inhibition was easily released by changing medium containing the drug to the drug free medium. Addition of MPB later than 5 hr, however, showed little inhibition of viral NAase induction in infected cells. Moreover, this drug as well as actinomycin D did not prevent the synthesis of NAase in infected HeLa cells with Newcastle disease virus. The mode of inhibitory action of MPB was very similar to that of actinomycin D excepting that MPB acted reversibly.