The pattern of staining for ANAE in lymphocytes, plasmocytes, monocytes and leukemic cells has been studied and the effect of NaF and E 600 on this staining reaction has been investigated. In lymphocytes the transition occasionally occurring between a dot-like (T type) and granular positivity may cause difficulties when using this reaction as a marker of resting T lymphocytes. In plasmocytes as well as in myeloma cells a number of coarse granules was present in most cells and in some cases even large blocks of the reaction product occurred. NaF and E 600 inhibits already in lower concentration (2 mM and 10 micronM respectively) the strong diffuse reaction in monocytes, the effect on other cells being negligible. Using higher concentrations the inhibitory effect of NaF (20 mM) was more pronounced in plasmocytes in comparison with T lymphocytes which on the other hand, were, in general more sensitive to E 600 (10 mM). Partially diffuse or dense granular pattern of ANAE was found in a number of hairy cells. "Lymphoblastic" leukemias in children most frequently displayed negative or fine granular reaction in a varying number of blasts and similar findings were noted in the majority of cytochemically undifferentiated leukemias in adults. In some of them, however, a similar pattern of ANAE as in myeloblastic leukemias with dispersed or partial diffuse positivity was observed. The significance of these findings is briefly discussed.