Abstract: This invention particularly discloses an improved immunoassay method for the specific detection of anti-rubella IgM antibodies. This method employs rubella antigens comprising rubella E1 and E2 envelope glycoproteins substantially free of capsid protein. Use of this antigen composition reduces or eliminates nonspecific protein-protein interactions leading to false positive results. A sample diluent comprising urea can also be used in the process to further reduce the occurrence of nonspecific protein-protein interactions. A diagnostic kit to aid in the detection of anti-rubella IgM antibodies is also disclosed.