Two studies were conducted to investigate the relationship between circulating levels of haptoglobin and α-1 acid glycoprotein (AGP) and growth in neonatal pigs. Circulating serum AGP, but not haptoglobin, was higher (P<0.001) in newborn runts than average-sized littermates. At 1 and 3 weeks, AGP and haptoglobin were similar among control and runt piglets. To determine the possible association between AGP and growth rate, blood was collected between the first and second day after birth in piglets from 10 average litters. Birthweight was positively correlated with growth rate through 21 days (linear regression correlation coefficient (CC), 0.43 (P<0.006); 0.299 (P<0.003) in males and females, respectively). Plasma AGP at birth was negatively correlated with growth (CC, -0.429 (P<0.006); -0.351 (P<0.01) in males and females, respectively). When AGP was calculated on a per kg birthweight basis, the CC with growth improved by 25 and 34% in males and females, respectively, compared with birthweight alone. Haptoglobin in blood was not correlated with growth. These data suggest that AGP at birth is reflective of growth conditions in utero or fetal maturation and may serve as an early predictive biomarker for pre-weaning growth rate.