The antiinflammatory cytokine IL-10 inhibits the production of multiple, diverse inflammatory mediators from activated macrophages and dendritic cells, a process requiring STAT3 activation. However, the mechanisms involved in the broad inhibitory effects of IL-10 are controversial. I eliminated the contribution of the major confounding variable to understanding the antiinflammatory response, the 3′ UTR region of inflammatory mediator genes, through knock-in mutation and analysis of the effects of IL-10 on transcription rate of inflammatory genes. IL-10 activates STAT3 to act indirectly by selectively inhibiting gene transcription independent of general effects on NF-κB or posttranscriptional mRNA processing through a process that reduces the overall transcriptional rate of specific genes.