BHK21 (clone 13S) cells of high (BHK-SH) and low (BHK-SL) passage number were infected with foot-and-mouth disease virus (FMDV) subtypes A24, A25 and C3. While the amount of virus specific RNA produced in BHK-SH cells was 25% of that in BHK-SL cells and the virion production was 27% (C3) to 53% (A24) lower, the synthesis of viral proteins was comparable, associated with an accumulation of procapsids in BHK-SH cells. The results suggest that changes in viral infection pattern with increasing BHK21 cell passage number should be considered in FMDV vaccine production.