The levels, isoforms and distribution of apolipoprotein A-IV (apo A-IV) were investigated in 127 term human umbilical cord sera. In addition, apo A-IV levels and isoforms were determined on the 3rd (n = 82) and 6th (n = 68) day following parturition and compared to apo A-I concentrations. Levels of apo A-IV were low in umbilical cord serum (5.7 +/- 1.9 mg/dl) as compared to adult serum (17.6 +/- 4.8 mg/dl). No difference was found between male and female neonates. The serum distribution of apo A-IV closely resembled that seen in the adult human. Apo A-IV concentrations dramatically increased during the first week of life reaching levels of 13.4 +/- 4.1 mg/dl on day 3 and 16.7 +/- 3.4 mg/dl on day 6 post-partum. During this time apo A-I levels did not change significantly (81.0 +/- 16.5 mg/dl in cord serum, 75.3 +/- 10.6 mg/dl and 84.2 +/- 14.5 mg/dl on day 3 and 6, respectively). Cord serum already exhibited the major serum apo A-IV isoforms seen in the adult. Isofocusing of apo A-IV also identified the known genetic polymorphism of apo A-IV. Among 127 cord sera studied we identified 109 homozygote normal patterns, apo A-IV (1-1), 16 heterozygotes, apo A-IV (1-2) and 2 individuals homozygote for the variant peptide, apo A-IV (2-2). We provide evidence that apo A-IV and apo A-I are differently induced in the human neonate during the beginning of the feeding period.(ABSTRACT TRUNCATED AT 250 WORDS)