Immune checkpoint inhibitors targeting PD-1 and CTLA-4 have shown clinical benefits in solid tumors, but their efficacy remains limited by low response rates, acquired resistance, and immune-related adverse events. These limitations have highlighted the need for alternative immune regulatory targets. LAG-3, a next-generation immune checkpoint co-expressed with PD-1, has emerged as an important therapeutic target. While the approved anti-LAG-3 antibody relatlimab primarily blocks LAG-3/MHC II binding, recent studies have identified FGL1 as an MHC II-independent ligand of LAG-3 that potently suppresses T-cell function. This finding supports the development of antibodies that selectively block the FGL1-LAG-3 axis. In this study, we generated antibodies specific to the D1-D2 domains of LAG-3 and reformatted them into chimeric antibodies to evaluate their blocking activity. Recombinant LAG-3 was expressed and purified for mouse immunization and hybridoma generation. ELISA screening and domain mapping identified three D1-D2-binding antibodies, and two clones were selected based on affinity. The selected antibodies were converted into chimeric antibodies containing a human IgG4 Fc region. BLI and ELISA confirmed high-affinity binding to LAG-3 at approximately 500 pM. Competitive binding assays showed dose-dependent inhibition of the FGL1-LAG-3 interaction by both antibodies, suggesting their potential as therapeutic candidates targeting the FGL1-LAG-3 axis.