AbstractThe mannan binding lectin (MBL) activates the complement system by the lectin pathway after the recognition of some structural motifs (saccharides) present on the surface of microorganisms. MBL has been mostly identified and quantified in human serum by ELISA or microparticle immunonephelometry assays. This article reports the MBL levels as assessed by a microparticle immunonephelometric assay in 76 human milk samples. Immunonephelometry was performed using skim‐milk samples diluted 20 times over a calibration range of 0.07–4.82 mg/L. MBL is indeed present in human milk and its concentration decreases significantly during development from colostrum (0.55±0.09 mg/L) to transitional (0.18±0.02 mg/L) and mature milk (0.17±0.02 mg/L). This innate molecule may be involved in the primary defenses of the mammary gland and the neonate, whose immune system is immature. The high levels observed during the first days of lactation support the hypothesis that this molecule plays a key role in limiting the colonization of the newborn gut by pathogens. J. Clin. Lab. Anal. 16:304–307, 2002. © 2002 Wiley‐Liss, Inc.