A homologous radioreceptor assay (RRA) has been developed for Insulin-like Growth Factor II (IGF II) using the human erythroleukemia cell line K562. These cells have binding sites for insulin and IGF-II but not for Insulin-like Growth Factor I (IGF I). All samples were dissociated and separated from binding proteins by gel filtration at acidic pH. In healthy adults the mean serum level of radioreceptor assayable IGF II (RRA-IGF II) and 95% confidence limits were 965 ng/ml and 717-1299 ng/ml, respectively. The mean level in GH deficient patients was significantly lower (p less than 0.001) compared with healthy subjects whereas no change was found in patients with acromegaly and uremia. Slightly lowered levels of RRA-IGF II were found in one patient with a tumor induced hypoglycemia.