Mannan‐binding protein (MBP) is a member of the collectins. These lectins are composed of polypeptide chains which contains a collagen‐like region and a calcium dependent carbohydrate recognition domain (named a C‐type CRD). MBP binds via the CRD to carbohydrate structures on microorganisms. MBP can activate the complement system when bound to carbohydrate. MBP is thus thought to play a role in the defence against microorganisms. The present report describes the ontogeny of MBP. The level of MBP increase during the first 3 months of life, at which time a stable level is reached, comparable to that seen in adults. On average, the level of MBP at term was 37% of the post 3‐month level. Measurements on infants born prematurely showed a 3‐fold increase in MBP concentration from the time of birth at around 30 weeks from conception to the 10th week of life.;