Tissue inhibitor of metalloproteinases-1 (TIMP-1), the major physiological matrix metalloproteinase inhibitor and a potent antimetastatic factor, also stimulates the growth of erythroid progenitors (erythroid-potentiating activity). We analyzed the relationship between the growth factor activity and protease inhibition by preparing purified TIMP-1 “knockout” proteins lacking in vitro antiproteolytic activity. The growth-stimulatory effect of these N- terminal TIMP-1 point mutants, as tested in an in vitro assay using erythroid precursors (erythroid burst-forming units) was equal to that of unmutated TIMP-1. A fully antiproteolytic C-terminal TIMP-1 truncation also stimulated growth in the erythroid burst-forming unit assay. The results indicate that the influence of TIMP-1 on erythroid precursor growth is independent of its ability to inhibit metalloproteinases. TIMP-1 is analogous to proteins that have both proteolytic and growth factor activity, such as plasmin, thrombin, and urokinase. However, TIMP-1 is novel in this regard because it is a metalloproteinase inhibitor. We show that the antiproteolytic and growth factor activities of the TIMP-1 molecule are physically and functionally distinct.