For the purpose to reveal the mechanism of tuberculous anemia in guinea pig, the author decided to apply the method devised by Seno et al. for the observation of ripening process of rabbit reticulocyte in vitro. As the ripening process of the guinea-pig reticulocyte has not yet been clarified, the author tried first to reveal the rinenine process of guinea-pig reticulocyte by means of this method for the basal experiment in the future observations and arrived at the following conclusion 1. Guinea-pig reticulocyte in the circulating blood has been proved to mature in test tube just as well as those of rabbit. 2. On the average, it requires 9 hours for the complete maturation and about 55% of the whole reticulocytes matures by this method. 3. By introducing these value into the formula proposed by Seno it has been calculated that the life span of erythrocyte of guinea pig is about 83 days.