We have used recombinant DNA technology to engineer a set of murine 3T3 cell lines that vary in the extent to which they express TIMP, tissue inhibitor of metalloproteinases, and we have found that both invasiveness and tumorigenic potential are conferred when TIMP production is impaired. These cell clones were produced by transfecting immortal Swiss 3T3 cells with plasmid constructs capable of expressing antisense TIMP RNA under the control of the mouse metallothionein promoter (Khokha & Denhardt, AntiCancer Res. 7: 653-660, 1987). The ability of these cells to invade the amnion membrane in vitro was dependent upon metalloproteinase expression (Yagel et al., JNCI 81: 768-775, 1989). Cells expressing TIMP at a reduced level acquired the ability to form tumors in nude mice (Khokha et al. SCIENCE 243: 947-950, 1989). These results suggest not only that TIMP controls the invasive character of the immortal 3T3 cell, but also that it determines cellular tumorigenic potential in the mouse. We presume that these phenotypes are conferred as a consequence of a net increase in extracellular matrix metalloproteinase activity resulting from the reduced quantities of TIMP secreted. The lag in formation of tumors by the cells down-modulated for TIMP production suggests that further changes in gene expression may be necessary. In support of this hypothesis, recent experiments indicate that the expression of genes encoding one or more matrix metalloproteinases is increased in cell lines derived from tumors that have developed from the engineered cells. Thus, in the immortal 3T3 cell line, TIMP has the properties of a tumor-suppressor gene, or anti-oncogene.