The concentrations of total protein (TP), albumin and globulin in the blood serum were investigated in seven trials with 203 piglets weaned at the age of three to four weeks. The lowest values were recorded in the newborn piglets just before colostrum intake. Two days later the TP level was three times higher and that of globulin almost five times higher. Later the levels began to sink (more intensively in globulin) and the decrease lasted until the end of the second month of age. Albuminaemia increased from birth until the end of study (the beginning of the third month) without any greater influence of colostrum intake and at a rate reduced by weaning. The albumin-globulin quotient, the lowest on the second day of life, increased step by step. The better-growing piglets mostly had higher albuminaemia and their hypoglobulinaemia reached a normal level sooner after weaning as compared with the tail-enders. This suggests that nutrition, body growth and albumin and globulin synthesis are interdependent in piglets. An increase in serum corticosteroids induced by two days of starvation or two administrations of adrenocorticotropic hormone, caused an insignificant increase in the concentration of albumin. Two hours after ACTH administration, the high level of corticosteroids was not observed to be accompanied by a change in serum proteins.