Mitogen‐induced interferon‐gamma (IFN‐gamma) gene expression was analyzed in human tonsil cells by titration of IFN‐gamma activity and by quantitation of IFN‐gamma mRNA. Expression of the IFN‐gamma gene can be superinduced extensively by two distinct methods: exposure to various inhibitors of translation, or to low doses of gamma‐irradiation. gamma‐Irradiated cells produce, after exposure to cycloheximide, up to 12‐fold greater amounts of IFN‐gamma activity. Within as little as 4 h after the addition of translation inhibitors, IFN‐gamma mRNA levels rise 3‐ to 5‐fold. Superinduction acts to increase the size of the wave of IFN‐gamma mRNA. Primary transcription of the IFN‐gamma gene does not increase in cells superinduced by cycloheximide, nor can superinduction be explained by stabilization of IFN‐gamma mRNA sequences. These findings show that, during normal induction, a labile protein acts post‐transcriptionally to repress the accumulation of mature IFN‐gamma mRNA sequences. The superinductive effects of cycloheximide and gamma‐irradiation on levels of IFN‐gamma are additive, suggesting that they affect different aspects of IFN‐gamma gene expression. Superinduction by gamma‐irradiation also has a post‐transcriptional basis and is consistent with the possibility that expression of the IFN‐gamma gene is normally controlled by the action of suppressor T cells. Even though the genes for human IFN‐gamma and for interleukin‐2 are both superinducible, a striking difference in the regulation of expression of these lymphokine genes is observed. Superinduction of IFN‐gamma mRNA is not due to superinduction of interleukin‐2.