A canine oral acanthomatous epulis cell line designated MCA-B1 was established in culture. Immunohistochemically the cells reacted intensely with anti-keratin antibody and mildly with anti-vimentin antibody. An ultrastructural examination of the cells confirmed their epithelial nature. The modal chromosome number was 72 and the cloning efficiency in soft agar culture ranged from 3.4 to 16.1 per cent with a population doubling time of nine hours. MCA-B1 has since been passaged more than 70 times and maintained in culture for more than a year. Most of the characteristics of the original canine tumour cells were retained in the isolated cell line.