Numerous studies have reported a correlation between the production of gelatinases A and B by cancer cells and invasive and metastatic potential. It has been suggested that the expression of gelatinase A (72-kDa type IV collagenase) is associated more closely with the metastatic phenotype of malignant cells in vitro and in vivo than that of gelatinase B (92-kDa type IV collagenase). We have established a rat bladder carcinoma cell line, MYU3L, which is tumorigenic and locally invasive but is not metastatic to the distal organs in nude mice. The MYU3L cell line secretes pro-gelatinase B but not any detectable level of pro-gelatinase A. We undertook the present study to determine whether over-expression of gelatinase A can affect the metastatic potential of MYU3L cells. We transfected MYU3L cells with an expression vector containing human pro-gelatinase A cDNA under the transcriptional control of the SR alpha promoter. Two stable transfectants over-expressing gelatinase A activity were isolated. We assessed the biological behavior of the transfectants by an orthotopic site (urinary bladder) inoculation and an i.v. injection in nude mice. Our results demonstrate that the induced expression of human gelatinase A enzyme markedly accelerates the metastatic phenotype of the rat bladder carcinoma cell line MYU3L. Our results suggest that gelatinase A produced by tumor cells plays a major role in the metastatic process.