endogenous somatostatinlike peptides of rat basophilic leukemia cells the tetradecapeptide somatostatin som 14 and a 28amino acid biosynthetic precursor som 28 are constituents of diverse neuroendocrine tissues that are released by noxious stimuli from a subset of sensory nerve endings and substantially modify the functions of basophils and mast cells somlike factors were detected initially in the fluid phase of suspensions of immunologically challenged rat basophilic leukemia cells rbl and were purified from ethanol02 m acetic acid 31 vv extracts of replicate portions of 3 x 109 rbl sephadex g25 columns resolved factors of over 10000 2000 to 4000 and 300 to 1200 daltons that are antigenically related to som 14 as assessed by a radioimmunoassay specific for som 14 only the two larger factors were detected by a radioimmunoassay for som 28114 which binds to preprosom and som 28 but not som 14 reversephase high performance liquid chromatography distinguished the two smaller som peptides of rbl from som 28 and som 14 respectively amino acid analyses showed major differences in composition between the 2000 to 4000 dalton som of rbl and som 28 picomolar to nanomolar concentrations of both of the smaller som peptides of rbl inhibited the igedependent release of histamine from basophils to the same extent as som 14 the finding of 3 to 5 ng of structurally unique somlike peptides per 108 rbl suggests that endogenous mechanisms analogous to those of specialized sensory neurons may regulate the expression of hypersensitivity