Heat shock factor 1 activates the promoters of heat shock genes at elevated temperatures through its interaction with heat shock elements. We have examined a new role for heat shock factor 1 in the repression of the prointerleukin 1beta gene in human monocytes responding to stimulation with lipopolysaccharide. Both exposure to elevated temperatures and heat-independent heat shock factor 1 expression repressed the transcription of the prointerleukin 1beta gene, and repression was strictly dependent on an intact consensus heat shock element in the prointerleukin 1beta promoter to which heat shock factor 1 bound. This is the first demonstration of heat shock factor 1 as a transcriptional repressor and suggests a role for the factor in the counter-regulation of cytokine gene transcription.