The in vitro clonal growth of T lymphocytes was examined in a patient with somatostatinoma over a period of 8 months. After phytohemagglutinin stimulation colony-forming unit T lymphocytes could not be detected as compared to healthy controls and patients with metastatic colon tumors. The somatostatin produced by this tumor was identified as somatostatin-14. Synthetic somatostatin-14 inhibits significantly the colony formation and the tritiated thymidine incorporation of human peripheral T lymphocytes obtained from healthy subjects, suggesting immunoregulative properties of this neuroendocrine peptide.