Interferon (IFN) production by human peripheral lymphocytes stimulated with M. pneumoniae was investigated. The hydrogen peroxide released from M. pneumoniae was responsible for the induction of IFN from lymphocytes, since horseradish peroxidase inhibited the IFN production and abrogated the activity of IFN production in the supernatant of M. pneumoniae. The antiserum neutralizing IFN alpha and IFN beta failed to neutralize partially interferon produced by lymphocytes. Treatment either with pH 2.0 or antiserum neutralizing human IFN gamma resulted in a partial reduction of interferon. These results indicate that interferon produced by human lymphocytes stimulated with M. pneumoniae includes both types of IFN gamma and IFN beta.