Feline calicivirus (FCV) can be typed by exploiting antigenic differences between isolates or, more recently, by the sequence analysis of a hypervariable region of the virus's capsid gene. These two methods were used to characterise FCV isolates from 20 vaccine failures which occurred after the use of a commercial, live-attenuated vaccine. Using virus neutralisation, the isolates showed a spectrum of relatedness to the vaccine; depending on the criterion adopted for identity, 10 to 40 per cent of them appeared to be similar to the vaccine virus. Using sequence analysis, the isolates fell into one of two categories; 20 per cent had a similar sequence to the vaccine (0-67 to 2-67 per cent distant), and the remainder had a dissimilar sequence (21-3 to 36-0 per cent distant). Sequence analysis identified one cat that appeared to be infected with two distinct FCVs. The serological and sequence-based typing methods gave the same result in 80 to 95 per cent of individual cases, depending on the criterion adopted for serological identity. It is suggested that molecular typing is a more definitive method for characterising the relatedness of FCV isolates.