Degradation of 14C-labelled type I collagen films by breast cancer cells. Breast cancer cells (105 cells/well) stimulated with TGFβ (10-10 M) were cultured for 24 h on 14C-labelled type I collagen under the following conditions: serum-free conditions; presence of 10% serum; 10% plasminogen (Plg)-depleted serum; serum-free medium supplemented with 2 μg/ml of human Plg. After 24-h incubation, collagen degradation was measured as described in Materials and methods. 8–800 mU (Ploug units) of pure human uPA in 1 ml of serum-free medium with or without 2 ug/ml of plasminogen, incubated as a control in parallel wells in the absence of cells, released 2–4% of the total radioactivity. This experiment was repeated twice. The results are expressed as percentage release of 14C. Each bar is the mean ± S.E.M of six wells. The stimulatory effects of plasminogen and TGFβ on breast cancer cell mediated 14C release were statistically significant ***P < 0.001 compared with the unstimulated controls and the HME cells.