The level, phenotypes, and isozyme distribution of adenosine deaminase (ADA) were determined in lymphocytes from patients with chronic lymphocytic leukemia (CLL). The ADA level in lymphocytes from patients with untreated CLL was consistently lower than in lymphocytes from normal subjects. No significant differences were found in the phenotype or isozyme distribution. In untreated patients, the ADA level was inversely correlated with the lymphocyte count and the percentage of bursa-equivalent (B) cells. After therapy, a diminution in the lymphocyte count was associated with an increase of ADA activity towards normal levels. The ADA levels were slightly higher in the thymus-derived (T) than in the B lymphocytes from normal subjects. The B cells had lower activity than T cells in patients with CLL. They also had a lower activity than the B cells from normal subjects. The ADA level was 2.3-fold higher in T cells from patients with CLL than in normal T cells. The decrease in ADA levels is an anomaly that is reversible and appears to be a reflection of the proliferation of abnormal B cells in this disorder.