Colostrum is a special type of milk produced in eutherian mammals during the end of pregnancy and during the 1st few days after birth. It supplies passive immunity to the offspring. The composition of colostrum and mature milk is compared in this study. In species with prenatal passive immunization (humans, baboons, and rabbits), immunoglobulin transfer via colostrum is of little importance and the difference in relative protein concentration between colostrum and mature milk can be small. In ungulates, on the other hand, colostrum has to supply the offspring postnatally with passive immunity and colostrum is relatively rich in immunoglobulin. Large differences between relative protein concentration in colostrum and milk can be observed in ungulates. Compositions of colostrum and milk thus reflect differences in immunoglobulin transfer.