Oronasal inoculation of minimal-disease cats with a virulent field isolate of canine coronavirus elicited a neutralizing antibody response that increased steadily over a two-month period, indicating in vivo amplification of the inoculated dose. A heterologous neutralizing antibody response to transmissible gastroenteritis virus could be detected three weeks after titers to canine coronavirus were first observed and was found to parallel the homologous response at a level approximately one log10 dilution lower. A modest anamnestic response detectable in both assays was observed after a second exposure to canine coronavirus. Cross-reacting antibodies to transmissible gastroenteritis virus were not detected by a kinetics-based enzyme-linked immunosorbent assay (KELA), however, until after cats had received multiple daily doses of canine coronavirus (third exposure). No clinical signs attributable to canine coronavirus were seen in inoculated cats, and virus could not be isolated from fecal or rectal swabs nor identified in fecal specimens by electron microscopy. Both virus-neutralizing and KELA titers were boosted by aerosol challenge with feline infectious peritonitis virus. Antibodies produced in response to canine coronavirus did not sensitize cats to subsequent feline infectious peritonitis virus challenge, but neither did they protect cats against the challenge dose.