Leukocyte alkaline phosphatase (LAP) activity was studied in 50 women at delivery and in the newborn after delivery. The foüowing findings were observed: (1) the LAP activity of newborns is lower than that of the mothers; (2) significant decrease in the LAP activity has been observed in newborns on the 4th day after delivery; (3) there are no significant differences between newborns from normal and prolonged labor; (4) immediately after delivery the LAP scores of male newborns are lower than those of females, but 4 days later these scores were reversed, and (5) 4 days after delivery, the LAP scores do correlate to the weight of the newborn.