Abstract: In a method of producing monoclonal antibodies against endothelial type plasminogen activator inhibitor and immunologically similar inhibitors myeloma cells are fused with antibody-producing cells obtained from mammals which have been immunized with said plasminogen activator inhibitor or with antibody-producing cells, which in vitro have been immunized with said plasminogen activator inhibitor and the hybridomas producing antibodies against the above mentioned inhibitor are selected. The antibody-producing cells are preferably spleen cells or lymph node cells, most preferably spleen cells, obtained from mice immunized with the above mentioned inhibitor.