Antibody 73 is a mouse IgG1 directed against an epitope overlapping with residues 27-45 of IGF2. It has an apparent affinity constant of 5.5 l/nMol and can be used at a dilution of approx. 1:10’000 for RIA of human serum. A half maximal displacement is achieved by 1.2- 1.3 ng IGF2 per 0.2 ml. The lowest concentration detectable is 0.3 ng IGF2 per 0.2 ml. No crossreactivity is observed with human insulin (50 ?g/200 ?l), proinsulin (1 μg/200 μl) and D-peptide of IGF2 (10μg/200μl). Antibody 73 recognizes IGF 2 of other species with following approximate relative potencies: guinea pig, human, mouse, rabbit, rat = 1.3:1:<0.4:<0.1: