The larger molecules of immunoglobulin G (LIgG) were found in ascites fluid containing monoclonal antibody against Clostridium botulinum toxin, and were purified by gel filtration and affinity chromatography. The purified material had the antigenicity and antibody activity similar to those of IgG obtained from the same ascites fluid. The molecular weight (Mr) estimated by gel filtration of the purified material was 250, 000, while LIgG molecules dissociated into a kind of molecule with Mr 245, 000 and nine kinds of the component molecules with Mr 220, 000, 188, 000, 152, 000, 135, 000, 105, 000, 75, 000, 67, 000, 52, 000 and 25, 000 in polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS-PAGE) in the absence of 2-mercaptoethanol (2ME). In SDS-PAGE in the presence of 2ME, the bands of Mr above 135, 000 disappeared and three bands of Mr 120, 000, 82, 000 and 67, 000 newly appeared besides the usual hesvy and light chains of IgG. These results suggest that hybridoma cells produce not only normal IgG but also LIgG.