A radioimmunoassay was developed using an antibody raised in rabbits against synthetic porcine PYY. This radioimmunoassay was used to detect PYY immunoreactivity in human intestinal extracts. Human colonic mucosa was extracted with acid, centrifuged and the supernatant concentrated by low pressure preparative reverse phase chromatography. A subsequent C-18 reverse phase HPLC step separated two peaks of PYY immunoreactivity. Each peak was purified by sequential steps of ion-exchange FPLC and reverse phase HPLC. In the final purification step single absorbance peaks were associated with PYY immunoreactivity. Microsequence, amino acid, and mass spectral analysis of the intact and tryptic fragments of the two peptides were consistent with the structures: YPIKPEAPGEDASPEELNRYYASLRHYLNLVTRQRY-amide [human PYY(1-36)] and--IKPEAPGEDASPEELNRYYASLRHYLNLVTRQRY-amide [human PYY(3-36)]. Human PYY(1-36) differs from porcine PYY only at position 3, with Ile instead of Ala, and position 18, with Asn instead of Ser. PYY(3-36) may differ in its biological activity from the intact peptide. Its high proportions in the colon suggest that it is released into the circulation where it could act as a partial antagonist of PYY(1-36).