Prolactin binding protein in human milk.

Milk samples at day 16 of lactation were from six mothers delivering infants at 28-30 weeks of gestation and six mothers delivering normal full term infants. These early morning samples were used in a prolactin binding assay to determine whether 125I-labelled human prolactin was selectively bound to a soluble protein fraction. After Sephadex G-100 column chromatography, 7 samples contained a protein fraction which bound human prolactin. Binding was not apparent in the remaining 5 samples. Prior binding of maternal prolactin, total binding capacity and differing free prolactin levels may be significant factors responsible for these observations.