alpha-Lactalbumin was purified from human milk and a competitive radioimmunoassay for measuring serum concentrations of human alpha-lactalbumin was developed. Human alpha-lactalbumin was not detected (less than 5 micrograms/l) in serum from adult men (n = 4), non-pregnant women (n = 6) or in serum from seven of eight formula fed infants. alpha-Lactalbumin was found in serum from pregnant women (19-130 micrograms/l, n = 4), cord blood (22-72 micrograms/l, median value 35 micrograms/l, n = 9), and from newborn non-fed infants (less than 1 day old) (less than 5-50 micrograms/l, median value 15 micrograms/l, n = 11). In breast fed infants the serum concentration of alpha-lactalbumin was highest in preterm infants (140-952 micrograms/l serum/l human milk/kg body weight, n = 4) and decreased in term infants successively with maturity (age 5-30 days: median value 85 micrograms/l serum/l human milk/kg body weight, n = 7; age 31-60 days: median value 43, n = 6; age 61-135 days: median value 12, n = 6). A human milk feeding to three infants one month of age gave serum peak values of alpha-lactalbumin after 30 to 60 minutes. We suggest that human alpha-lactalbumin is a suitable marker for investigating macromolecular absorption in physiological and pathological conditions.