14 patients with chronic myelocytic leukemia were evaluated immunologically; no difference was found in mean lymphocyte percentage and absolute number between patients and healthy subjects. 4 cases (28.5%) showed decreased percentage of T lymphocytes, while only 2 cases (14.2%) had decreased absolute T lymphocyte values. PHA transformation was decreased in 57% of the patients, Spontaneous transformation in the short-term cultures exceeded the normal range in 65% of the cases. All patients skin tested were found to be reactive. Most of the patients had defective cellular immune response in vitro, probably related with a qualitative defect in T lymphocyte subpopula-tions. It cannot be completely excluded that part of the observed lymphocyte depression was due to the busulfan.