Ascitic fluid from a patient with carcinoma of the pancreas was fractionated by ammonium sulfate precipitation. The fraction precipitated between 25 and 50% saturation of ammonium sulfate was sequentially chromatographed on Sephadex G-200 and Sepharose 6B. A macromolecular fraction (greater than 10(6) daltons) obtained was found to react with both antihuman IgM and antiserum to carcinoembryonic antigen (CEA). This fraction was further purified by adsorption with protein A-Sepharose CL-4B and chromatography on DEAE-Sephacel. The purified macromolecular fraction had a sedimentation value of 28S as determined by ultracentrifugation. Upon dissociation of the purified macromolecule at pH 2.3 and purification of the dissociated components on Sepharose CL-2B and BioGel A 1.5M, a 19S protein and a 5S protein were recovered. The 19S protein showed a complete line of identity with a reference human IgM when reacted with antihuman IgM in gel diffusion, whereas the 5S protein showed a partial immunologic identity with colon CEA against anti-CEA. These results indicated the existence of an IgM-containing macromolecular complex with an anti-CEA cross-reactive substance in the extracellular fluid of human pancreatic cancer.