Evidence is presented that peptides may occur in aggregated form. Addition of 125I-Tyr-DSIP to serum resulted in four peaks after gel filtration chromatography on a column of Sephadex G-25. One of the peaks (C) eluted at the same position as the labeled peptide standard. Two Peaks (A and B) eluted before the standard and one (peak D) afterwards. The first peak (A) eluted at void volume, a position expected for labeled peptide bound to protein. The other two peaks (B and D), corresponding to smaller molecular size material, were greatly reduced by addition of glacial acetic acid or the chelating agent 1,10-phenanthroline before or even after mixing of the peptide with serum. Iron was one of the ions found to interact with 125I-Tyr-DSIP, and chromatography of a mixture of ferric chloride and peptide without serum resulted in the additional formation of peak B. A substantial portion of peaks A, B, and C (but not D) reacted with a specific DSIP antibody, indicating the presence of intact peptide. The results are consistent with the concept that peptides may occur in multiple forms.