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Adenosine deaminase (ADA) activity has been assessed in lymphoid cells of 23 patients with acute lymphoblastic leukaemia (ALL) in order to attempt a further characterization of ALL cells in addition to the well known cytochemical and immunological T and B lymphoid cell markers. ADA activity did not show any correlation with the immunological characterization of the patients investigated; in fact a wide range of ADA activity was observed with levels ranging from 0 to 32 U in T-ALL patients and nearly similar values (from 1.8 to 36 U) in the group of non T--non B ALL cases. Normal values ranged from 2 to 5 U (mean 2.9 U; s.d. +/- 0.8). Some cytochemical patterns (acid phosphatase and PAS) appeared well correlated with T markers of lymphoid cells, whereas they showed no significant relationship with ADA activity.

The labelling index (LI) of myelocytes (M) after flash labelling of normal human bone marrow cells with [3H]-thymidine ([3H]TdR) is always lower that the LI obtained for myeloblasts (MB) and for promyelocytes (PM). This fact can be interpreted in two ways: it may mean that the duration of the G1 phase of the cell cycle is longer in M than in MB or PM, or it may mean that the proportion of cells in cycle, i.e., the growth fraction (GF), is lower in the M population than in MB or PM. the evolution of the LI and of the mean number of grains per cell was monitored in [3H]TdR-labelled normal bone marrow during in vitro incubation for 50 hr. the generation time, measured by the halving time of the mean number of grains per cell after flash labelling, was similar for M to that for MB and PM. During continuous labelling, the LI of MB and PM reached 1 and the LI value for M never rose to more than 50% of the values recorded for MB and PM after 30 hr. These findings give support to the second hypothesis, i.e., a lower GF in the M population. Good correlation was found between the LI of M and the proportion of mature polymorphonuclear cells in the bone marrow of normal subjects and of patients with chronic benign neutropenia or hyperleucocytosis. Variations in the M growth fraction could be a medium-term (2-3 days) regulatory factor in granulocyte production.

Eosinophilic leukocytes may accompany a great variety of disorders and different types of acute leukemias. The most striking morphologic feature of eosinophils is their specific granules, but morphology alone often is insufficient to differentiate normal from abnormal eosinophils. Cytochemically, the eosinophils were considered "normal" when they did not contain alkaline phosphatase, chloroacetate esterase, toluidine blue metachromasia, Astra blue positivity, and specific PAS-positive granules, but did have peroxidase and cyanide-resistant peroxidase activities, Sudan black positivity and moderate naphthol-AS esterase or alpha-naphthyl esterase and acid phosphatase positivities. In seven cases of acute leukemias (two acute myeloblastic and five myelomonocytic), in contrast with their normal behaviour, the eosinophils show "abnormal" cytochemical positivities consisting of chloroesterase activity, PAS and Astra blue positivities of the specific granules, toluidine blue metachromasia, and cyanide-resistant peroxidase of a few specific granules. Cytochemical investigations may provide additional criteria for evaluating the abnormality of the eosinophilic cell in leukemias.