Nine live virus veterinary vaccines from six sources were found to be contaminated with mycoplasma. The vaccines were for use in canine, feline and avian species, and 53 batches of the products were at fault. The isolates were identified as Mycoplasma hominis, M. arginini, M. orale, M. hyorhinis and M. gallinarum. Investigation of the contamination rate of other batches or other products from the same source in some cases helped to determine the source of infection. Mycoplasma contaminants can be considered important not only because of their role as pathogens but also because they may indicate that insufficient care has been taken during vaccine manufacture or quality control.