The acid-labile sub-unit (ALS) of insulin like growth factor binding protein complex in biologically pure form is described.ALS has a molecular weight between 80 115 kd as determined by SDS polyacrylamide gel electrophoresis, run under reducing conditions; and a partial N-terminal amino acid sequence as follows: Gly AspProGlyThrProGlyGluAlaGluGlyProAlaCysProAlaAlaCys- Ala wherein the first amino acid may be Gly or Ala. Also described are methods of producing ALS, compositions containing the in-vivo IGF protein complex, methods of detecting ALS in body fluids, recombinant nucleic acid sequences encoding ALS, and expression vectors and host cells containing such nucleic acid sequences.