Sera from 39 preterm and 35 full-term infants with hyperbilirubinemia, undergoing phototherapy continuously for 48 h, were assayed for unbound bilirubin by the peroxidase oxidation method. Assessment was done to determine the influence of therapeutic exposure to light on the serum unbound bilirubin concentrations in preterm and full-term infants. At 24 and 48 h after initiation of phototherapy, the unbound/total bilirubin ratios, as well, as the serum total and unbound bilirubin levels showed statistically significant declines of values in both preterm and full-term infants. It is concluded that, following phototherapy, the serum levels of unbound bilirubin decline more conspicuously than the decrease in serum total bilirubin levels. © 1981 S. Karger AG, Basel.