We have demonstrated previously that treatment of the phorbol ester PMA-primed THP-1 human macrophage-like cells with interferon-ç (IFN-ç) in vitro induced time and dose-dependent increases in steady-state levels of cathepsin B (CB) mRNA. The present study was undertaken to investigate the mechanism of that increase. In vitro nuclear transcription (nuclear run-off) assays of CB gene expression were performed with purified nuclei from IFN-ç-treated or untreated THP-1 cells. These assays showed transcription to be increased approximately three-fold by IFN-ç in the PMA-primed THP-1 cells. Studies with á-amanitin indicated that the half-life of CB mRNA is prolonged after PMA and IFN-ç treatment, by more than 90%. Therefore, the elevated CB mRNA level results from a combination of IFN-ç-induced increase in the transcription rate of the CB gene and stabilization of the corresponding transcripts. The IFN-ç-mediated increase in CB gene transcription and steady-state mRNA level was blocked by á-amanitin or cycloheximide, suggesting the involvement of RNA polymerase II and the requirement of de novo protein synthesis.