Lymphocytes from a case of B-cell chronic lymphocytic leukemia (CLL) were obtained in a highly purified state from a therapeutic leukapheresis preparation. The CLL lymphocytes showed a fine, scattered, granular pattern of nonspecific esterase cytochemical reactivity with either alpha-naphthyl acetate (alpha NA) or alpha-naphthyl butyrate (alpha NB) substrate as opposed to the more focal pattern of control (T) lymphocytes. Nonspecific esterase of CLL lymphocytes and normal control lymphocytes was equally resistant to inhibition by fluoride ion. Extractable nonspecific esterases from the CLL lymphocytes and from purified normal T lymphocytes were indistinguishable in regard to specific activity, substrate specificity, pH optima, and zymogram profiles on polyacrylamide gel electrophoresis at pH 9.5 and pH 4.0. Zymograms of alpha NA esterase and alpha NB esterase prepared by isoelectric focusing were also similar, with no unequivocal differences. These results are consistent with recent reports that B lymphocytes contain detectable nonspecific esterase and suggest that the B lymphocytes from this case of CLL contained a constellation of isoenzymes similar to that of normal T lymphocytes. This is interpreted as a reflection of the close kinship of these cells.