Odorant-binding proteins (OBPs) in insects occur within olfactory sensilla, and are thought to transport chemical stimuli to receptors on dendrites of sensory neurons. Until recently, knowledge of OBPs in insects was limited to moths and Drosophila. We discovered an antennal-specific protein (Lygus [lineolaris] antennal protein, LAP) with a unique N-terminal sequence in the true bug, Lygus lineolaris. We localized LAP to antennae, determined its molecular weight (16 kDa), and showed that while it was expressed in nymphal antennae, its levels dramatically increased in adults concurrent with increases in numbers of olfactory sensilla and electrical responses to odors. In our current study, we used immunological techniques to demonstrate in more detail that LAP occurs only in antennae, and to show its expression within Lygus species. LAP was expressed more in male antennae than in antennae of females for the Lygus species examined. Anti-LAP did not recognize antennal proteins of two other genera of bugs. Immunocytological studies showed LAP primarily within the sensillar lymph of type 1 and type 4 sensilla on antennae. These observations strongly suggest LAP to be an OBP, and our discovery and characterization of OBPs in true bugs provides a third order for use in the study of evolution of OBPs in insects.