Specific pathogen free cats were infected by two feline calicivirus isolates of different plaque type, an extra-large plaque (ep) former and a minute plaque (mp) former. A comparison was made of the disease produced when these isolates were administered by either aerosol or direct intranasal instillation. With both isolates, aerosol infection produced lesions and gave rise to virus replication throughout the respiratory tract. The effects of intranasal infection were more confined to the upper respiratory tract and oropharynx. By both routes of infection the disease produced by the mp virus was clinically and pathologically less severe than that produced by the ep virus.