Although matrix metalloproteinases (MMPs) are implicated in breast cancer progression, the contribution of MMP-1 and MMP-3 to this process, has not been thoroughly investigated. Matrix metalloproteinases (MMPs) are important at several points during multistage neoplastic progression. Immunohistochemistry (Strept-ABC-HRP method) and in situ hybridization were performed to detect MMP-1, MMM-3 proteins, and MMP-3 mRNA, respectively, in 77 infiltrative breast carcinomas. MMP-1, MMP-3 protein, and MMP-3 mRNA detection were analyzed in parallel with clinicopathologic features (menopausal status, histological type, nuclear and histological grade, stage) and the immunohistochemical reactivity of estrogen (ER), progesterone (PR) receptors, and c-erbB-2 oncoprotein in breast carcinomas. Statistical analysis was performed using the multiple linear regression test. Immunoreactivity for MMP-1 and MMP-3 was observed in 59 of 77 (77%) and 22 of 77 (28.5%) breast carcinomas and was evaluated separately in cancer cells and in stromal fibroblasts. MMP-3 mRNA was detected in 72 of 77 (93.5%) carcinomas exclusively in stromal cells within the tumors or in the marginal portion of tumors. MMP-1 protein immunoreactivity in stromal fibroblasts but not in cancer cells showed a statistically significant correlation with tumor stage (P=.04). MMP-1 reactivity either in stromal or in cancer cells showed a statistically significant inverse correlation with PR expression (P=.04 and P=.04, respectively). MMP-3 protein immunoreactivity in cancer or stromal cells and MMP-3 mRNA expression was not associated with the clinicopathologic features studied. MMP-3 mRNA was detected more often in ductal carcinomas. These results indicate that MMP-1 may contribute to breast cancer invasiveness. Furthermore, they suggest differential functions for MMP-1 and MMP-3 in breast cancer progression.