Erythrocyte and leukocyte ralues are recorded front samples of blood from 240 fetuses and piglets aged from 30 to 144 days after conception. The erythrocyte count and packed cell volume increased during the first half of fetal life. There was a sharp transient decrease in these values post-natally. Haemoglobin values were constant during the second hall of fetal life and then, after an initial post-natal drop, they increased. A similar transient reduction in leukocyte population followed a marked perinatal leukocytosis. Lymphocytes exceeded the neutrophil series at all ages except in early fetal life and during the immediate perinatal period
