Two batches of experimental media containing 10% milk whey (sweet or sour) and 1% bovine serum supported the growth of BHK-21 cells in serial passages. The cell yield was greater than 2 x 10(6)/ml at 48 h. The cells were susceptible to inoculation with foot and mouth disease (FMD) virus type 'A' and the virus titres obtained were comparable with those in Eagle's MEM-G with 10% serum. The use of milk whey could reduce the consumption of serum required for the growth of FMD virus in BHK-21 cells by up to 90%.