Findings with six cytochemical reactions demonstrable in normal and leukaemic lymphocytes were reviewed. The two methods which are presently of greater diagnostic value are the acid phosphatase (AP) and alpha-naphthyl acetate esterase (ANAE) reactions. AP has a definitive role in the diagnosis of acute and chronic T-cell leukaemias, where a strong positive reaction helps to distinguish them from most B-cell lymphoproliferative disorders. New findings concerning the ultrastructural localization of this enzyme are presented. ANAE is of value in distinguishing T-lymphocytes (positive localized reaction) from B lymphocytes (negative reaction) and the T micron from the T gamma subpopulation of T-lymphocytes, a positive reaction demonstrable only in the T micron cells. Other reactions reviewed were PAS, beta-glucoronidase, hexosaminidase and alkaline phosphatase.