β2-Microglobulin concentrations were determined in 78 maternal and 21 neonatal colostrum samples collected on various days post partum. Maternal concentrations were highest (about 50-fold those observed in normal blood) on the 1st day, falling to a steady level similar to that of normal blood by the end of the 1st week. Neonatal levels, on the other hand, increased as secretion proceeded. The highest value (about 5-fold that of normal serum) was noted in a 22-day-old subject. It is suggested that β2-microglobulin production is characteristically associated with colostrum secretion and that colostrum itself has the highest concentration of the protein.