Studies on the blastogenesis of PHA stimulated lymphocytes of the cord blood of newborns, and the vencus bloods of the 2∼6 month old normal infants, normal adults and chromic lympoid leukemia patients of the Koreans and Israeli were performed. The results are summerized as follows: 1) Lymphocytes from cord blood of newborns showed slightly higher transformation rate while the infants showed significantly low comparing with that of normal adults. 2) Sex difference in transformation rate is not significant. 3) Mean values of transformation rates in Koreans are slightly higher than that of Israeli, however the Koreans disclosed more individual variations. 4) 3H-thymidine incorporation rate in 2∼6 month old infants is markedly low while the blast-transformation rate is relatively high, indicating that the DNA synthesis rate of PHA stimulated lymphocytes in this age group is very low comparing that of newborns and adults.