Summary.When reticulocytes in vitro are suspended in physiological salt water and incubated at 40° the reticulocytes disappear only very slowly.The rate of disappearance can be accelerated considerably by adding liver extract to the salt solution or when the reticulocytes are suspended in plasma.The youngest and most unripe types disappear first. The disappearance thus must be due to a maturation of the reticulocytes into “adult” erythrocytes and this process must be accelerated by some unknown ripening substances found in plasma and liver extract.The ripening process follows the monomolecular equation.The monomolecular constant is proportional to the concentration of ripening substances in the suspending solution.The monomolecular constant increases with increasing temperature.When reticulocytes from the same animal are used, the monomolecular constant found at different times is the same if the same concentration of ripening substances is used in the experiments.The monomolecular constant can vary ± 25 per cent when reticulocytes from different animals are used. The proportion between two tested solutions is, however, always the same, independent of the animals