Piglet leucocytes and plasma cortisol were studied at birth, one, two, five, 10 and 20 days after birth; one, 12 and 21 days after weaning, and in adults. Plasma cortisol concentrations fell rapidly from very high levels at birth to adult values by five days old. These values were then maintained throughout the rest of the study period. The number of neutrophils rose rapidly after birth, declined during late lactation and then rose to adult levels after weaning. The number of lymphocytes rose steadily throughout the study period to reach a peak 12 days after weaning at levels double that of adults. There was no significant variation in the proportion of B lymphocytes between ages, the values fluctuated between 11 and 18 per cent. There was no significant difference in the proportion of B lymphocytes between piglets at birth and adults. However, the number of B lymphocytes was significantly depressed at birth, and elevated during weaning, when compared with adults. The leucocyte results were verified by flow cytometry which provided an objective estimate of values such as the proportion of lymphocytes and B lymphocytes. In addition, this method revealed that in the two days after birth there was a transient decrease in the proportion of B lymphocytes bearing a high density of surface immunoglobulin.