Recently, we described a novel fibroblast-restricted monoclonal antibody (mAb AS02) that recognizes a membrane-bound antigen. Characterization and isolation of the corresponding antigen showed that mAb AS02 recognized a protein on human fibroblasts that is highly homologous or identical to human Thy-1 antigen (CD90). Partial amino acid sequencing of the corresponding mAb AS02 antigen and comparison with known proteins revealed a 100% homology of the sequenced peptides to the human Thy-1 antigen. Cross-immunodepletion studies with mAb AS02 and an anti-Thy-1 antibody confirmed these results. Utilizing two-dimensional (2D) gel electrophoresis of fibroblast cell extracts and purified antigen, mAb AS02 and the anti-Thy-1-antibody recognized identical protein spots. Furthermore, we demonstrated many identical biochemical properties of the corresponding AS02 antigen and Thy-1 antigen, such as the molecular weight of the core protein and deglycosylation products and the detection of a GPI anchor. In functional assays, the attachment of fibroblasts to collagen I and fibronectin was increased after incubation of fibroblasts with mAb AS02. Therefore, the Thy-1 antigen appears to be involved in the regulation of the adherence of human dermal fibroblasts.