During the last few years, advances in the care of low-birth-weight and preterm neonates has stimulated research on the best dietetic program to improve survival and to reduce handicap incidence. At present, fortification of human milk with artificial formulas is the most usual dietetic solution. As yet, however, little is known about the composition of milk from mothers giving birth prematurely. The aim of this study was the quantification of different proteins in human milk during the lactation period. By use of an electrophoretic method, lactoferrin (LF), alpha-lactalbumin, beta-casein, and lysozyme concentrations were measured in milk from mothers delivering normally (TM) or prematurely (PM). LF concentration in milk from TM presented higher values in the very first days and a fast decrease to d 10. After d 10, the concentration reached a plateau. In milk from PM, the LF concentration in the first days was lower than for TM. Similar profiles of alpha-lactalbumin, beta-casein, and lysozyme concentrations were found in milk from TM and PM. A general higher variability in PM samples was observed both between different mothers and for the same woman during the lactation period. Lactation profiles for four human milk proteins are described here. No significant difference was observed (apart from LF in the very first days) between preterm and term milk samples, confirming the unsuitability of unfortified breast milk for preterm neonates.