The leukocyte migration inhibition (LMI) technique was used to investigate the status of cellular sensitization in human patients suffering from chronic myeloid leukemia (CML). Peripheral leukocytes from CML patients in remission (remission-leukocytes) and from normal donors (normal-leukocytes) were examined for their susceptibility to extracts prepared from myeloid type of leukemic cells. In further experiments to define the specificity of the reactions observed, both normal-leukocytes and remission-leukocytes were also examined in the presence of similar extracts prepared from (a) normal leukocytes, (b) lymphocytic leukemia cells, and (c) 9 biopsies of solid tumors of different types ( 2 melanomas, 2 sarcomas, and 5 carcinomas). Extracts of myeloid leukemic cells prepared by hypotonic sodium chloride (NaCl) and hypertonic potassium chloride (KCl) extraction procedures showed significant inhibitory activity preferentially directed against the migration of remission-leukocytes. The KCl-extracts showed greater inhibitory activity and exerted it in a dose-related manner. The activity persisted in extracts of leukemic cells obtained serially from CML patients before and during treatment, and during subsequent relapses. Similar extracts of normal leukocytes and of 9 biopsies of solid tumours failed to show such discriminatory behavior against the two types of leukocytes tested. However, the susceptibility of the remission-leukocytes was not uniquely directed towards the extracts of myeloid leukemic cells. Their migration was also inhibited in a considerable fraction of the tests carried out with the extracts of lymphocytic leukemic cells. Inhibition of migration of remission-leukocytes by extracts of “allogeneic” leukemic cells indicated common antigenic component(s) shared by leukemic cells.