Residual cellular material present in foot-and-mouth disease (FMD) vaccines has been implicated as a possible cause of hypersensitivity in cattle. This study was designed to identify and characterize the potentially allergenic components involved. After fractionating baby hamster kidney (BHK) cell lysate, by preparative electrofocusing and gel filtration chromatography, the eluates which were potentially allergenic for cattle were identified by passive cutaneous anaphylaxis (PCA) and PCA-inhibition methods. The potential allergen identified had a molecular weight of approximately 43 000 daltons, sedimentation coefficient of 3·65 S20 and isoelectric points between pH 5·37 and 6·25. Based on these results an immunogenic FMD vaccine was produced which failed to provoke clinical sensitization after repeated inoculations. This information should enable manufacturers to identify, monitor and subsequently control the levels of any such potentially allergenic material present in current vaccines produced using BHK cells.