A one-step chromatographic procedure was used to isolate rapidly mouse IgG monoclonal antibodies (mAbs) (IgG1, IgG2a, IgG2b) contained in ascites fluids and Fab fragments contained in papain-treated mAb suspensions. Chromatographic separations were performed on an anion-exchange Mono Q column connected to a fast protein liquid chromatographic (FPLC) system. Detection of mAb or their antigen binding fragments (Fab) in eluted peaks was performed using sodium dodecyl sulphate-polyacrylamide gel electrophoresis together with a silver or a Coomassie Brillant Blue R 250 staining technique and solid phase radioimmunoassay with 125I-labelled sheep anti-mouse antibodies directed against total immunoglobulins. Rapid assessment of the purity of isolated mAbs and their Fab fragments was performed by gel permeation chromatography on a TSK G 3000 SW column. Mouse mAbs and their Fab fragments were rapidly isolated (25 min), in a functionally active state, to a high degree of purity on the FPLC-Mono Q system compared to the time taken by other techniques.