Monocytes and macrophages play an important role in both innate and acquired immunity. Cytokines like interleukin-4 (IL-4) produced by T-cells and mast cells can influence monocyte and macrophage actions. A previous study showed that IL-4 stimulation leads to over 2000-fold up-regulation of the MAO-A enzyme in U937 cells, but not the MAO-B isoform. This study examines transcriptional regulation of MAO-A by IL-4. Electrophoretic mobility shift assays revealed that the transcription factor SP1 binds to probes from the MAO-A promoter region, especially in cells stimulated with IL-4 for 3 days. This suggests SP1 plays a role in upregulating