Changes in the lactoferrin and lysozyme concentration of human milk during lactation were determined by microparticle-enhanced nephelometric immunoassays of 360 milk samples collected from 64 lactating volunteers. These 360 samples were colostrum from days 1 to 5 postpartum (142 samples), transitional milk from days 6 to 14 (106 samples), and 112 mature milk samples obtained from days 15 to 28 (34 samples), from days 29 to 56 (50 samples) and from days 57 to 84 postpartum (28 samples). The concentration and percentage of lactoferrin vs. total protein were found to be significantly higher in colostrum (5.8 g/L, 27%) than in transitional milk (3.1 g/L, 22%) or day 15 to 28 mature milk (2.0 g/L, 19%), then increased in day 29 to 56 mature milk (2.2 g/L, 22%) and day 57 to 84 mature milk (3.3 g/L, 30%). The concentration of lysozyme decreased from colostrum (0.37 g/L) to transitional milk (0.27g/L) and day 15 to 28 mature milk (0.24 g/L), then increased in day 29 to 56 mature milk (0.33 g/L) and was highest in day 57 to 84 mature milk (0.89 g/L). The percentage of lysozyme vs. total protein was found to be always rising during lactation: colostrum, 2%; transitional milk, 2%; days 15 to 28, 2%; days 29 to 56, 3%; and days 57 to 84 mature milk, 8%.