This study was carried out to evaluate in vitro the beneficial effects observed in various aggressive fibromatoses (mediastinal, retroperitoneal, paraneoplastic fibrosis and desmoid tumors) after treatment with progesterone. Primary cultures of fibroblasts were prepared from fetuses of Swiss strain mice. Continuous fibroblast lines LM and Vero were also used. Moreover, cultures of nonfetal human fibroblast from skin and lung were employed. Epithelial tumor cell line HeLa was used as a control. All cultures were incubated with various doses of progesterone at concentrations from 1.4 × 10––4 to 1.4 × 10––3M. Human cells and monolayers of fetal murine fibroblast were submitted to the action of medroxyprogesterone solution at the same concentrations as used for progesterone. Other steroids (estrone, estriol, testosterone and prednisolone) were used at the identical concentrations in the same vehicles. Progesterone affected all lines of fibroblasts studied and destroyed them within either 2–4 or 24–48 h depending on the steroid concentrations used. Medroxyprogesterone had a comparable effect on human cell lines and monolayers of fetal murine fibroblasts provided that the same ratio between the hormone concentration and the time of exposure was maintained. With higher medium concentrations shorter times of incubation were required for the destruction of fibroblast. However, to observe a degree of lysis similar to that elicited by progesterone, it was necessary to use 4 times higher concentrations of medroxyprogesterone. No effect on HeLa epithelial cells was observed nor were the controls affected by the steroids or diluents used at the appropriate concentrations. Results from incubation studies using monolayers of murine fibroblast and 14C-progesterone suggested that the cells were destroyed by the progesterone and not by a bio-product of its metabolism.