A cotton rat model of experimental human parainfluenza virus type 3 (PIV-3) infection was used to examine the efficacy of FHN, a novel chimeric glycoprotein which contains the extracellular regions of the fusion (F) and haemagglutinin-neuraminidase (HN) glycoproteins of PIV-3. The FHN protein was expressed in insect cells using a baculovirus vector system. FHN vaccination resulted in induction of neutralizing antibodies, was completely protective at doses of 100 ng, and was superior to vaccination with secreted forms F and HN proteins, or mixtures of the F and HN glycoproteins. In addition, FHN immunization induced lymphoproliferative responses in mice which were directed against both the F and HN glycoproteins. Fusion of the F and HN proteins into a single chimeric glycoprotein appeared to enhance the protective immune response compared to that elicited by the individual glycoproteins or mixtures of the two glycoproteins.