We measured neonatal apo A-I and apo B by ELISA, and apo(a) by RIA, in capillary blood spotted onto filter paper in samples also used for routine neonatal screening in 1032 consecutively born babies. In the 2- to 5-d-oId babies with birth weights >2.0 kg (n = 919), mean ± SD levels of apo A-I and B were 0.48 ± 0.19 g/L and 0.24 ± 0.14 g/L of whole blood, respectively. The apo A-I levels were affected by birth weight (negatively) and by age at sampling (positively). The apo B levels were affected positively by both variables, and girls had higher levels than boys (p < 0.01). These variables accounted for 3.5 and 6.2% of apo A-I and apo B variability, respectively (p < 0.001). The apo(a) levels (mean ± SD, 20 ± 23 U/L; median, 14 U/L, n = 1032) were unaffected by these factors. After adjustment for these variables, apo A-I levels were nearly normally distributed, whereas those of apo B were still positively skewed. The apo(a) distribution was strongly positively skewed and 1.2% of babies had levels above the equivalent of 25 mg/dL of lipoprotein(a) in serum. Our study shows that blood spots can be used to estimate apo A-I, apo B, and apo(a) levels in neonates, and establishes normal ranges. The results suggest that the apo(a) gene is expressed during the 1st postnatal week and that levels are independent of birth weight and apo A-I and B concentrations. They also define the effects of birth weight and age at sampling on neonatal apo A-I and B levels.