Lactoferrin was measured immunologically in human colostrum and in milk from 2 to 197 days after parturition. Lactoferrin decreased during the first two weeks, paralleling a decrease in total and in whey protein nitrogen. Then the curve sloped more gradually. Colostrum, 2 to 5 days postpartum, 8 samples; transitional milk, 6 to 10 days, 10 samples; mature milk, 11 to 60 days, 43 samples; and mature milk, 61 to 197 days, 25 samples contained (mean±standard deviation), .49 ±.06, .45±.08, .21±.05 and .16±.03g of lactoferrin per 100ml, respectively. From nitrogen distributions, acrylamide gel electrophoresis and immunological quantitation, human whey protein contained about 35.0% lactoferrin 46.4% α-lactalbumin, 10.6% serum albumin and 8.0% remaining proteins (globulins). Normal human milk, 11 to 60 days postpartum, contained about, 44g casein, .21g lactoferrin, .28g α-lactalbumin, .06g serum albumin and .05g remaining proteins (globulins). The casein to whey protein ratio is about 1:1.4.