Substantially pure modified .beta..sub.2 -microglobulin (m.beta..sub.2 m) of the formula I ##STR1## wherein R.sub.1 is 24-amino acid residue, with the sequence Ile-Gln-Arg-Thr-Pro-Lys-Ile-Gln-Val-Tyr-Ser-Arg-His-Pro-Ala-Glu-Asn-Gly-Lys-Ser-Asn-Phe-Leu-Asn, R.sub.2 is a 30-amino acid residue with the sequence Tyr-Val-Ser-Gly-Phe-His-Pro-Ser-Asp-Ile-Glu-Val-Asp-Leu-Leu-Lys-Asn-Gly-Glu-Arg-Ile-Gly-Lys-Val-Glu-His-Ser-Asp-Leu-Ser, R.sub.3 is a 20-amino acid residue with the sequence Trp-Ser-Phe-Tyr-Leu-Leu-Tyr-Tyr-Glu-Phe-Thr-Pro-Thr-Glu-Lys-Asp-Glu-Tyr-Ala, R.sub.4 is a 19-amino acid residue with the sequence Arg-Val-Asn-His-Val-Thr-Leu-Ser-Gln-Pro-Lys-Ile-Val-Lys-Trp-Asp-Arg-Asp-Met, X is Phe, Phe-Ser, or Phe-Ser-Lys, and Y is Asp, Lys-Asp, or Ser-Lys-Asp is disclosed. The presence of the protein in body fluids is a diagnostic and/or prognostic marker for the development of a variety of disorders such as different types of cancer and diseases involving the immune system. Also disclosed are specific anti-m.beta..sub.2 m antibodies used for the detection and quantification of m.beta..sub.2 m.