The authors have investigated the cytokinetic behaviour of 23 cases of non-Hodgkin B cell lymphomas. In all cases diagnosis was established on the basis of histological, cytoimmunological and cytochemical data. The study, carried out with the aid of autoradiographic techniques, indicates that the various histological subclasses are characterized by a different metabolic and kinetic behaviour: the aggressiveness of the malignant lymphoma is closely related to the rate of growth of the lymphomatous cell populations, as assessed by the mitotic index and the labelling index with 3H-thymidine. The study of the synthesis and metabolism of RNA provides not only useful information on the energy requirements necessary for the neoplastic growth, but, when employed in association with immunohistochemical techniques for the detection of intracytoplasmic Ig synthesis, it defines more accurately the functional characteristics and the differentiation potentialities of the neoplastic populations in different cases.