The sex steroid hormone progesterone (Pg) is critically involved in the development of the mammary gland, and it also is thought to play a role in breast cancer progression. However, the effect of Pg on malignant phenotypes is not fully understood in breast cancer. We previously reported that in Pg receptor (PR)-positive T47D breast cancer cells, Pg was able to counterbalance the stimulatory effect of estrogen or serum on proliferation and on expression level of Id-1, which generally stimulates cell proliferation and inhibits differentiation. Conversely, metastatic MDA-MB231 breast cancer cells lack PR and express high levels of Id-1 constitutively, and Pg showed no effect on Id expression, proliferation, and invasion in these cells. However, after introducing PR (either PR-A or PR-B) into MDA-MB231 cells, Pg inhibited the expression of Id-1 mRNA drastically. PR-transfected MDA-MB231 cells exhibited less proliferative activity after Pg treatment than parental or control MDA-MB231 cells, an effect which correlated well with reduction of Id-1 mRNA. This inhibitory effect on proliferation was accompanied by p21 up-regulation and c-myc down-regulation. Moreover, Pg-treated PR transfectants showed significant morphologic change, appearing more flattened and spread out than control ethanol-treated cells. Boyden chamber invasion assay revealed that PR-transfected MDA-MB231 cells also lost most of their invasive properties after Pg treatment. Zymographic analysis revealed that Pg drastically inhibited matrix metalloproteinase-9 (MMP-9) activity in cells transfected with either PR-A or PR-B. To determine whether Id-1 could act as a key mediator of the effects of Pg, we prepared cells transfected with Id-1 and PR. The morphologic change and p21 up-regulation still were observed after Pg treatment. However, c-myc down-regulation was not observed; the proliferative and invasive activities were mostly recovered; and MMP-9 down-regulation could not be detected anymore. From these observations, we conclude that either form of the PR is sufficient to reduce the malignant phenotypes on treatment with Pg and that Id-1 plays an important role as a mediator of the effects of Pg on breast cancer cell proliferation and invasion.