Four acid hydrolases, acid phosphatase (AP), alpha-naphthyl acetate esterase (ANAE), beta-glucuronidase, and N-acetyl-beta-glucosaminidase, were determined cytochemically in peripheral blood lymphocytes from 50 patients with B and T chronic lymphocytic and prolymphocytic leukemias (CLL, PLL) and related disorders. Strong positive reactions were characteristic of the T-cell leukemias while the reactions were weak or negative in B-CLL and B-PLL. Differences in the cytochemical profile of T-CLL and T-PLL were noted. In both, beta-glucuronidase and N-acetyl-beta-glucosaminidase were positive; these enzymes are therefore good cytochemical markers of the chronic T-cell leukemias. AP and ANAE gave different results according to the disease process; AP was strong in T-CLL and variable in T-PLL, while ANAE was strongly positive in T-PLL, but weak or negative in T-CLL. The findings in T-CLL, a proliferation of T gamma lymphocytes, were similar to those of normal T gamma cells. In T-PLL, the findings did not relate to the membrane phenotype as defined by monoclonal antibodies.