The chemical characteristics of odorant-binding proteins (OBPs) are summarised and compared with those of other types of binding proteins involved in chemical communication. The recent finding of multiple forms of OBPs in the same animal species could suggest an odour discriminating role for these proteins. In this respect, the vertebrates' olfactory system is similar to other chemoreception systems, such as those present in insects for detecting pheromones and general odours, bacterial chemotaxis and taste.