Macrophage-conditioned media-enhanced breast fibroblast invasion requires matrix metalloproteinase (MMP)-9. Primary human macrophages were treated +/- estrogen (10-10 M) in 2% charcoal-stripped serum-containing media and 24 hr-conditioned media were collected. Primary human breast fibroblasts were seeded in serum-free media into Matrigel invasion chambers in triplicate. Chemoattractants were conditioned media (CM) from macrophages treated with estrogen (10-10 M) +/- the indicated amount of MMP9 inhibitor. (A) 24 hr post-seeding, invaded cells were stained with toluidine blue; scale bar = 500 μm. (B) Data represent average number of invaded cells +/- SD; four areas were measured per chamber. *P < 0.01.