Milk donated by mothers of term infants may not be of optimal nutrient concentration if used to feed low birthweight infants (LBW), but since many milk constituents have functions other than classically 'nutritional' ones it is, in theory, qualitatively superior to artificial formulae. We describe a system for bulk preparation of a human milk protein concentrate which can be used in powder form to adjust the nutrient content of human milk to any specified protein/calorie content. Donor human milk is separated into skimmed milk (0.01g protein/g, 0.34kCal/g) and fat (0.01g protein/g, 2kCal/g). Skimmed milk is concentrated fourfold from 16-20L starting volume by a commercial ultrafiltration apparatus which contains2.5sq M of polysulfone membrane having a nominal molecular weight cut-off of 10,000. The resulting liquid concentrate (protein 0.04-0.05g/g) is lyophilised to a dry powder (protein 0.4g/g, energy 4kCal/g). Skimmed milk, protein-rich powder and fat are stored separately at -20°C. A microcomputer program has been developed which sepcified the weight of each fraction required to produce a human milk of any protein/calorie content desired. The human milk fractions can therefore be utilised to provide a feed of any composition required. Studies of the absorption and utilisation of milk constituents processed in this way are in progress.