The absorption of alpha-lactalbumin (ALA) and bovine beta-lactoglobulin (BLG) was investigated in 23 healthy preterm infants with gestational ages of 32 to 36 wk. The concentrations of ALA and BLG in serum after a milk feeding were measured at intervals during the first 8 mo of life. We used a time-resolved fluoroimmunoassay to measure the proteins. Measurable amounts of ALA were found on d 7 after birth, and at 1, 2, 3, 5, and 8 mo in 23 of 23, 13 of 18, 13 of 18, six of 17, eight of 16, and five of 13, respectively, of the infants tested; median serum levels of ALA at the respective ages were 120 (range, 19-2598), 16 (range, 0-177), 5 (range, 0-40), 0 (range 0-3), 0.8 (range 0-38), and 0 (range, 0-22) micrograms/L serum/g ALA given/kg body wt, respectively. The rate of decline in ALA absorption was comparable among the infants. Tests for BLG were begun after the introduction of cow's milk. At 2, 3, 5, and 8 mo of age BLG was detected in two of 7, two of 9, eight of 10, and two of 12, respectively, of the infants tested, where median levels in positive cases were 13, 17, 15, and 3 micrograms/L serum/g BLG given/kg, respectively. The amounts of absorbed ALA and BLG were 10(-5) to 10(-3) of the oral dose. Serum levels of ALA or BLG did not depend on the gestational age of the infant. Few of the infants had any detectable absorption of either protein shortly after weaning.(ABSTRACT TRUNCATED AT 250 WORDS)