Extracts of a carcinoid tumour, resected from the mid-portion of the ileum of a patient with no symptoms of endocrine disorder, were associated with a high concentration of substance P-like immunoreactivity. Using reverse-phase high performance liquid chromatography and antisera specifically directed against the C-terminal and N-terminal regions of substance P and against the N-terminal region of physalaemin, the following components were isolated and identified: substance P and its oxidised form, [pGlu5]substance P-(5-11) peptide and its oxidised form, and the oxidised form of physalaemin. The identity of tumour substance P with the undecapeptide was confirmed by amino acid analysis and high performance ion-exchange chromatography. In vitro incubation of tumour tissue from a lymph node metastasis from the same patient with [3H]leucine resulted in incorporation of radioactivity into newly synthesised substance P.