WE have been examining extensively the in vitro multiplication of oil-induced, rabbit peritoneal histiocytes by means of autoradiography. The cells were incubated on flying coverslips in Leighton-tubes. containing 40 per cent homologous serum in Tyrode solution in the presence of 1 μc. tritiated thymidine/ ml. In general, little incorporation occurred within the first 48 hr.; but then an increasing number of cells became labelled.