The time-course of changes in the levels of albumin, α-fetoprotein (AFP), α₁-protease inhibitor (α₁-antitrypsin), α₁-acid glycoprotein, fetuin, haptoglobin, transferrin, IgG and the major acute-phase protein (Pig-MAP) in the blood sera of pigs during the first days and weeks of life was investigated by quantitative radial immunodiffusion. The serum of newborn pigs before suckling was characterised by a very low concentration of total proteins (approximately 25 mg mL^–1), low levels of albumin and transferrin and the lack of immunoglobulins. In contrast, α₁-acid glycoprotein and fetuin are present at high levels (approximately 12 and 5 mg mL^–1 respectively). The results of the present study show that the piglets undergo a very rapid metabolic maturation with regard to serum proteins, evolving from a characteristic ‘fetal’ pattern to an ‘adult’ one. We have paid special attention to the evolution of haptoglobin and Pig-MAP, which are two important acute-phase proteins in pigs. The evolution of serum levels of these proteins suggests that piglets must overcome a moderate acute-phase situation during the first week of life.