Malignant astrocytomas are highly invasive tumors which infiltrate diffusely into regions of normal brain. The degradation of the extracellular matrix (ECM) by matrix metalloproteinases is thought to be one of the most important steps in the process of tumor invasion. However, the activity of most matrix metalloproteinases (MMPs) can be modulated by simultaneously secreted inhibitors (tissue inhibitors of metalloproteinases, TIMPs). We have previously shown that an imbalance between the levels of MMPs and TIMPs may be essential in the determination of the invasiveness of certain human malignant astrocytoma cell lines. To determine if the up-regulation of TIMP genes and gene products could modulate the invasiveness of human malignant astrocytoma cells, in the present study we have transfected a highly invasive astrocytoma cell line, SF-188, with an expression vector carrying a full-length TIMP-1 cDNA. The parental SF-188 astrocytoma cell line overexpresses the 72-kDa and 92-kDa type IV collagenases with little expression of TIMPs-1 and -2. Following transfection with TIMP-1, SF-188 astrocytoma clones expressed the 0.9 kb TIMP-1 message by northern analysis, and produced a 21 kDa metalloproteinase inhibitor by reverse zymography. The stable TIMP-1 SF-188 transformants demonstrated morphological changes and diminished growth rates in soft agar when compared to controls. The invasion of successfully TIMP-1 transfected astrocytoma cells across matrigel-coated filters was significantly decreased over controls. These results suggest that upregulation of TIMP-1 expression in SF-188 astrocytoma cells has decreased their in vitro invasive potential.