The odorant-binding protein (OBP) is an abundant, soluble protein of subunit molecular mass 18 000 daltons in the rat, synthesized in the lateral nasal gland, from where it is secreted into nasal mucus. The protein binds odorants of various structural classes such as pyrazines, terpenoids and aromatics. Molecular cloning and sequence analysis of several cDNAs for rat OBP indicate that the protein is significantly homologous to α2u-globulin and a superfamily of proteins that are believed to transport hydrophobic ligands. These data suggest that OBP transports odorants in mucus, possibly by a mechanism of facilitated diffusion. The proposed transport of odorants by OBP is analogous to the transport of molecular oxygen by hemoglobin.