KL-6 in serum and bronchoalveolar lavage fluid has been reported to be a sensitive marker indicating the activity of fibrosing lung diseases. The molecule is clustered in MUC1 mucin according to the findings of immunohistochemical and cytometric studies. To elucidate the pathogenic role of KL-6 in fibrosing lung disease, we characterized its biochemical properties and examined whether purified KL-6 is chemotactic for human fibroblasts in vitro using modified Boyden chambers. Biochemical properties of purified KL-6 were similar to those of other MUC1 mucins previously reported. KL-6 promoted the migration of 5 of 5 human lung fibroblasts and 3 of 4 human skin fibroblasts. Checkerboard analysis revealed that KL-6 was chemotactic as well as chemokinetic. Though platelet-derived growth factor, fibroblast growth factor, or fibronectin were also chemotactic for fibroblasts in the experimental system, only fibronectin augmented KL-6-induced chemotaxis. These observations indicate that KL-6 is one of the chemotactic factors for most fibroblasts and that the increased KL-6 in the epithelial lining fluid in small airways may cause the intra-alveolar fibrosis in fibrosing lung diseases.