Macrophages stimulate primary breast fibroblast proliferation. Primary human monocytes/macrophages were isolated, differentiated, treated +/- estrogen (10-10 M) for 24 hr and conditioned media were collected. Primary human breast fibroblasts were placed in 2% charcoal-stripped serum-containing media 24 hr prior to treatment +/- estrogen or with macrophage-conditioned media +/- estrogen for three days, then trypsinized and counted. Data represent four independent experiments with different fibroblasts and macrophage isolations used for each assay. *P < 0.01. ϕ, macrophage conditioned media; E2, estrogen treatment.