We investigated the usefulness of serum type IV collagenolytic activities and gelatinase levels as diagnostic markers of metastasis in the animal model of spontaneous lung metastasis by FITC-labeled type IV collagen degradation assay and zymographic analyses. High-metastatic RCT(+) and low-metastatic RCT(-) clones were used in the present study. The mean serum type IV collagenolytic activity in the RCT(+) group started to increase from two weeks after hind limb amputation, and was 0.45 and 1.29 unit/ml at three and four weeks. These values were significantly higher than those in the control group (p < 0.01 at three weeks; p < 0.001 at four weeks). A correlation between the number of lung nodules and serum type IV collagenolytic activities in the RCT(+) group was found (r = 0.89, p < 0.001). Zymographic analyses indicated that 105-kD gelatinolytic activities of the RCT(+) group were higher than those of the RCT(-) group at three and four weeks. Thus, type IV collagenolytic activities and serum gelatinase levels might be valuable markers for the detection of metastasis.