BACKGROUND: Human milk contains a lipase stimulated by bile salts (bile salt stimulated lipase, BSSL) which is important for fat digestion in the new born period. It is not present in cows' milk or formulae. The reported molecular mass of human milk BSSL has varied between 90 and 120 kD. The aim of this study was to investigate the discrepancies in reported molecular mass and to determine the existence and incidence of molecular forms in preterm and term milk. METHODS: BSSL was purified from 40 human milk samples by heparin sepharose chromatography. Twenty samples were donated between 3-79 postnatal days following preterm deliveries and twenty were donated between 4-175 postnatal days following term deliveries. Molecular forms of BSSL were characterised by SDS-PAGE and gel permeation chromatography. RESULTS: Approximately 35% of the purified milk samples contained two molecular forms of BSSL of variable molecular mass. The most common combinations were 98 and 120 kD (n=6), or 105 and 120 kD (n=4). The molecular mass of the single molecular form was either 115 kD (n=20) or 120 kD (n=6). CONCLUSION: The number of molecular forms present was not related to maternal age, blood group, gestation, or length of lactation, and the number of forms present remained constant following different pregnancies in the same individual. The specific activity of the purified BSSL was similar whether one or two molecular forms were present and whether from preterm or term milk. This study demonstrates that BSSL is heterogeneous with respect to molecular mass and number of molecular forms, and indicates that preterm babies fed their own mother's milk are unlikely to be disadvantaged with respect to fat digestion as BSSL in preterm milk appears to be very similar to that secreted in term milk.