immunohistochemical detection of somatostatin receptor subtypes sst1 and sst2a in human somatostatin receptor positive tumors although in situ hybridization has been used to examine the distribution of messenger rna for somatostatin receptor subtypes sst in human tumors the cellular localization of sst1 and sst2a receptors has not been reported in this study we describe the cellular localization of human sst1 and sst2a receptor proteins in both cryostat and paraffinembedded sections of 25 human tumor tissues using two recently developed polyclonal antibodies six somatostatin ss receptor ssr positive tumors two gastrinomas three carcinoids one pheochromocytoma and one ssr negative tumor renal cell carcinoma selected by positive and negative ssr autoradiography respectively were studied by both immunohistochemistry and western blot analysis the six ssr positive tumors expressed sst2a while 4 of 5 expressed sst1 as well the ssr negative tumor did not express either sst1 or sst2a western blot analysis of wheat germ agglutinin purified membrane proteins confirmed the presence of the sst1 and sst2a glycosylated receptors the paraffinembedded sections gave best information with respect to the subcellular localization sst1 immunoreactivity was observed both on the membrane and in the cytoplasm while sst2a showed predominantly membraneassociated immunoreactivity this subcellular distribution of sst1 or sst2a receptors was confirmed in paraffinembedded sections of 8 additional intestinal carcinoids 5 gastrinomas and 5 pheochromocytomas sst1 receptors were detected in 7 out of 8 carcinoids in all gastrinomas and in 4 out of 5 pheochromocytomas while 6 out of 8 carcinoids all gastrinomas and 3 out of 5 pheochromocytomas expressed sst2a receptors in conclusion sst1 and sst2a receptors show a differential subcellular localization in human ssr positive tumors the use of ssr subtype selective antibodies to detect the subcellular distribution of ssr subtypes in individual tumor cells is an important step forward to understand more about the pathophysiological role of the different ssr subtypes in human tumors