In two patients with advanced Hodgkin's disease the DNA content, the mitotic index and the in vitro tritiated thymidine labeling index of the neoplastic cells found in peripheral blood, bone marrow and, in one case, in ascitic fluid have been determined. Cytologically the tumor cells were classified into three groups: atypical mononuclear cells, Hodgkin (H) cells and Reed-Sternberg (R-S) cells. Two populations with clearly different kinetic features were recognized. The first one was that of atypical mononuclear cells which exhibited diploid DNA content and proliferative activity of moderate degree: tetraploid cells were, however, observed more frequently than expected. The second population grouped together H and R-S cells, had tetraploid modal DNA content and very high proliferative activity. Hodgkin cells had tetraploid or slightly greater DNA content while R-S cells exhibited also DNA values as high as octoploid and sometimes greater. It can be postulated that by endomitosis some atypical mononuclear cells give origin to H cells and that these assume the monstrous features of R-S cells while increasing their DNA content during the DNA synthesis phase.