We investigated the effects of arginine on the multiplication of herpes simplex virus type 1 (HSV-1) and the potential of arginine as an antiherpetic agent. Arginine suppressed the growth of HSV-1 concentration-dependently. Inhibition of HSV-1 by arginine leveled off at 50-60 mM, although the higher concentration was not suitable as an antiviral agent due to cytotoxicity. 'Time of addition' experiments revealed that arginine was particularly effective when added within 6 h post-infection (h p.i.), suggesting that the reagent sensitive step is in the early stages of the infection. A one-step growth curve of HSV-1 in the presence of 30 mM arginine revealed that: i) the latent period was significantly extended, ii) the rate of formation of progeny infectious virus decreased and iii) the final yield of progeny virus decreased to 1%. The addition of arginine at 8 h p.i., after the completion of viral DNA replication in the virus multiplication, allowed the normal formation of progeny virus in the subsequent 4 h, confirming that arginine does not directly interfere with the formation of progeny infectious virus. In addition, arginine also inhibits several RNA viruses.