3-Methyleneoxindole (MO), an oxidation product of the plant auxin indole-3-acetic acid, can selectively inhibit the replication of herpes-, mengo-, polioviruses, and Sindbis virus. The antiviral action of MO, a sulfhydryl binding compound, is neutralized by 2-mercaptoethanol if the latter is added soon after exposure of infected cells to MO. If addition of 2-mercaptoethanol is delayed, the antiviral action of MO appears to be irreversible. Data are presented which indicate that the antiviral action of MO is not mediated by interferon.