The specific activity of cyclic AMP phosphodiesterase was measured in lymphocytes isolated from the blood of normal subjects, from patients with chronic lymphocytic leukemia and from tonsil tissue. The mean specific activity of cyclic AMP phosphodiesterase in the lymphocytes from patients with untreated chronic lymphocytic leukemia was lower than that in lymphocytes from the blood of normal subjects or from tonsils. Cyclic AMP phoshpodiesterase levels did not correlate with differences in B[bone marrow derived]- and T[thymus derived]-cell lymphocyte subpopulations or with peripheral blood lymphocyte counts.