Somatostatin (SOM) is a neuroendocrine tetradecapeptide that suppresses specific functions of differentiated T-cells and antibody-producing cells. The Jurkat line of human leukemic T-cells and U266 IgE-producing human myeloma cells bound [I-Tyr11]SOM specifically. The maximal level of specific binding was attained by 1-2 h at 22 degrees C for both types of cells and reversed by 70-85% within 2-3 h after the addition of excess nonradioactive SOM. Computer-assisted Scatchard analysis of the competition curves revealed two classes of binding sites for both cells. An average of 144 and 1295 high affinity receptors per Jurkat and U266 cells had a Kd value of 3 pM and 5 pM, respectively, whereas a large number of low affinity sites had Kd values of 66 nM and 100 nM. The affinity of the analogs somatostatin 28, [I-Tyr11]SOM, and [D-Trp8, D-Cys14]SOM for Jurkat and U266 cell lines, relative to SOM, suggested a degree of specificity similar to receptors on neuroendocrine cells.