The virus-receptor interaction is critical for establishing infection in host cells. The interaction (i) allows the virion to attach specifically to the host cell membrane, (ii) is often required to permit the process of infection further downstream, and (iii) strongly influences viral tropism and pathogenesis. Feline calicivirus (FCV) is a common feline pathogen. Infection is generally asymptomatic or causes mild upper respiratory signs and oral ulceration; morbidity is high while mortality is low. During the last decade however, there have been outbreaks of extremely virulent viruses that cause systemic disease and result in mortality rates as high as 50%. One objective of these studies was to characterize in vitro properties of FCV isolates of high and low virulence and to identify in vitro correlates to virulence. FCV, a small nonenveloped virus containing a positive sense RNA genome, is known to bind two cellular receptors, feline junctional adhesion molecule A (fJAM-A) and ?2,6 sialic acid. A second objective was to elucidate the details of the FCV-fJAM-A interaction by determining the regions of fJAM-A required for FCV binding and selecting FCV mutants resistant to neutralization by soluble fJAM-A. Analysis of FCV in vitro properties demonstrated that highly virulent isolates were able to spread more efficiently in tissue culture than other isolates investigated. Highly virulent isolates were also more susceptible to neutralization by soluble fJAM A than other isolates. Lastly, sequence analysis of the FCV structural protein did not identify conserved sequence unique to highly virulent viruses. Mutagenesis studies identified that while the binding of FCV to fJAM-A requires the extracellular membrane-distal loop, all domains of the protein were required for successful infection. Selection of FCV mutants resistant to receptor neutralization and measurements of protein hydrophobicity provided genetic and biological evidence indicating that FCV undergoes a significant conformational change upon interaction with fJAM-A. This work shows that FCV interaction with fJAM-A results in changes to the capsid likely to be important for subsequent steps in infection, and suggests a determinant of virulence in FCV is accelerated entry post-binding.