Chromatographically purified human IgA1 lambda, dimer. Purified human IgA1 lambda may be used as a reference antigen, calibrator, coating protein and blocking agent in a variety of immunoassays including immunodiffusion, immunoelectrophoresis, ELISA, Western blotting, dot-immunobinding assays (DIBA), haemagglutination, and cell-binding assays. Chromatographically purified.