Exposure of human monocytes to Chlamydia pneumoniae resulted in a significant enhancement of matrix metalloproteinase (MMP) 1 and 9 production following stimulation with tumor necrosis factor alpha and granulocyte monocyte-colony stimulating factor. The effect of C pneumoniae on monocyte MMPs was mediated through the induction of prostaglandin E, These findings may have implications for atherosclerotic plaque rupture.