The collectins are a group of mammalian lectins containing collagen‐like regions. They include mannan binding protein, bovine conglutinin, lung surfactant protein A, lung surfactant protein D, and a newly discovered bovine protein named collectin‐43. These proteins share a very similar modular domain composition and overall 3‐dimensional structure. They also appear to play similar biological roles in the preimmune defense against microorganisms in both serum and lung surfactant. The close evolutionary relationship between the collectins is further emphasized by a common pattern of exons in their genomic structures and the presence of a gene cluster on chromosome 10 in humans that contains the genes known for the human collectins. Studies on the structure/function relationships within the collectins could provide insight into the properties of a growing number of proteins also containing collagenous regions such as Clq, the hibernation protein, the α‐ and β‐ficolins, as well as the membrane acetylcholinesterase and the macrophage scavenger receptor.