Wild type PAI-1 cloned with a cysteine residue at the N-terminus (patent pending). The active fraction is >95percent pure and >95percent active as determined by titration with HWM tc-urokinase and SDS PAGE. The protein is supplied in buffer with 1 mM DTT to prevent dimerization. To label with thiol reactive dye, buffer exchange into degassed 0.05M Sodium Phosphate; 0.1M NaCl; 1mM EDTA; pH 6.6 by desalting column. Collect into 10 fold molar excess of maleimide or iodoacetamide dye and incubate for 1 hour at room temperature. To preserve PAI-1 activity minimize the percentage of DMSO/DMF in the reaction. Remove free dye by desalting, ammonium sulfate precipitation, or dialysis.