SummaryThe serum LDH activity and the isozyme patterns in cord blood and the healthy individuals in different age were investigated during the development and the following results were obtained.1) LDH activityThe mean values in each group were as follows: 490.30 u/ml in cord blood, 623.60 u/ml in early newborns (1–3 days), 575.45 u/ml in newborns (4–7 days), 395.00 u/ml in infants (1–6 m), 371.00 u/ml in infants (6–12 m), 342.00 u/ml in young children (1–6 y), 298.24 u/ml in school children (6–15 y) and 212.00 u/ml in adults.Marked elevations of the activity were proved in the newborn group, especially 1–3 days after birth.After the newborn period, the activity gradually decreased to the adult level.No significant differences were obtained between the male and the female in each group.2) LDH isozymeThe distribution dominance of the LDH isozymes in all groups were indicated in the following order: LDH‐2>LDH‐1>LDH‐3>LDH‐4>LDH‐5.The LDH isozyme pattern in cord blood showed the specific pattern considerably different from those in other groups.The LDH isozyme patterns in newborn serum showed the statistically significant increase of LDH‐3, LDH‐4 and LDH‐5 as compared with those in other age groups and the alteration was marked in the early period of the newborn, 1–3 days after birth.3) The possible mechanisms of the changes of the serum LDH activity and the alterations of the