We have reported that down-modulation of tissue inhibitor of metalloproteinases (TIMP) by means of antisense RNA converts non-tumorigenic Swiss 3T3 cells into malignant cells capable of forming metastasizing tumors in nude mice [Science 243:947 (1989)]. We now describe changes in the expression of specific genes associated with tumor progression of two lines down-modulated with TIMP, LA1 and LA7. Six independent variant cell lines, generated from different primary tumors produced by LA1 and LA7, lacked (like LA1 and LA7) many characteristics of typical transformed cells. However, their tumorigenicity in nude mice was enhanced; tumors appeared with a shorter lag (1-3 weeks versus 8-10 weeks for the parental clones, LA1 and LA7) and grew very rapidly. Increases, substantial in some cases, in the expression of a cysteine proteinase, cathepsin L, and metalloproteinases homologous to rat transin (stromelysin) and transin-2 were characteristic of these variant clones. The mRNA levels encoding the transformation-associated secreted phosphoprotein (osteopontin) and the calcium-binding protein calcyclin were also augmented. No evidence for gene amplification was found, and we did not detect any change in the mRNA levels of the proto-oncogenes that were examined. These novel cell lines represent a new paradigm for the transformed cell. Our data suggest that a reduction in TIMP secretion enhances the cell's oncogenic capacity by altering the extracellular environment in a way conducive to further changes in gene expression necessary for tumor progression.