Freezing and thawing of solutions of [125I] human growth hormone (hGH) in phosphate buffer promotes the conversion of significant amounts of the hormone to high molecular weight material as seen by gel filtration on Sephadex G100. Storage at -20 C for one week may cause additional conversion. Rapid freezing and storage at -70 C fail to prevent the conversion. There is virtually no conversion if the [125I] hGH is stored at 4 C for periods of at least one week. We believe that this conversion represents aggregation of [125I] hGH monomers. The aggregate is not dissociated by incubation in 5 M guanidine. It is bound by anti-hGH antibody to a much smaller extent than is monomeric [125I][125I] hGH, so the presence of significant amounts of the aggregate in [125I] hGH tracer could seriously interfere with radioimmunoassay of hGH. To maintain homogeneous [125I] hGH monomer, we recommend storage of solutions of the radioiodinated hormone at 4 C. (Endocrinology92: 1795, 1973)