The serum lectin, mannan-binding lectin (MBL) (also denoted mannan-binding protein or mannose-binding protein, MBP) has been identified in mammals (humans, monkey, cow, rabbit, mouse and rat). Upon binding to carbohydrates on the surface of microorganisms, MBL mediates activation of the complement system, leading to killing of the microorganism. MBL thus exerts a role in the innate immune defence. We have described the isolation and partial characterization of an analogous protein in chicken serum. Oligonucleotides based on the N-terminal sequence of this protein were used in a reverse transcription-polymerase chain reaction (RT-PCR) with chicken liver RNA as template. The PCR product was sequenced and found to encode part of the NH2 terminus of chicken MBL. A perfect match probe was synthesized and used to screen a chicken liver cDNA library. The isolated clones carried a cDNA insert of 1692 bp with an open reading frame of 714 bp encoding a mature protein of 238 amino acids including a signal peptide of five amino acids. The deduced amino acid sequence agrees with those determined by conventional amino acid sequence analysis of the peptides except for four residues. We have compared the deduced primary structure of chicken MBL with the mammalian analogues. The phylogenetic analysis indicates that the gene duplication leading to two different MBL forms in mammals occurred after the split from birds and reptiles. This concurs with the finding of only one form of MBL in chickens.