Immunostaining for vasostatin I distinguishes between ileal and lung carcinoids. 
Although chromogranin A (CgA) is a recognized marker of neuroendocrine tumours, little is known about the distribution of the CgA-derived peptides, vasostatin (VST) I or II, in these tumours.  Rabbit polyclonal antiserum was raised to a fragment of VST I and used to immunostain sections (5 microns) of wax-embedded tumour tissue.  Immunoreactivity (IR) was detected using swine anti-rabbit fluorescein secondary antibody and sections were viewed by fluorescence microscopy.  Of 24 tumours from patients with lung carcinoids, one was weakly positive, while 23 of 26 ileal carcinoid tumours were immunoreactive.  Metastatic deposits from patients with ileal carcinoids also tended to be immunoreactive (9/10).  The difference in IR between lung and ileal carcinoid primary tumours did not appear to be related to the metastatic potential, since appendiceal tumours, which seldom metastasize, also tended to be immunoreactive (4/6) for VST I. The strongest IR was recorded in two patients with flushing as a result of ileal carcinoids; five other 'flushers' with ileal carcinoids were also immunopositive for VST I-like IR.  By contrast, patients with flushing as a result of lung carcinoids were immunonegative for VST.  In conclusion, VST I-like IR may assist in the identification of a secondary deposit from an unknown primary site.