Abstract: Objective: To study the relation between the change of ribonuclease inhibitor(RI) and malignant proferations in different malignant degree human breast cancer cell lines MCF-7、MDA-MB-231、SKBR-3.Methods: Cell profigeration was determined by MTT assay;The cell cycle was measured by flow cytometry(FCM);the expression of RI mRNA was examined by semi-quantitative RT-PCR;immunohistochemistry was used to detected the protein expression of RI.Results: Human breast cancer cell lines MCF-7 cell malignant proliferation.S period was less than the propotion of human breast cancer cell lines MDA-MB-231,SKBR-3(P<0.05-P<0.01),and the expressions of RI mRNA and protein in the human breast cancer cell lines MCF-7 were higer than that of human breast cancer cell lines MDA-MB-231、SKBR-3(P<0.01).Conclusions: RI and human breast cancer cells malignant profigeration growth has certain relevance.