Five species of domestic animals were examined immunohistochemically and the light-chain ratios of the immunoglobulins produced by plasma cells analysed. Forty dogs, 11 cats, 10 horses, 11 cattle and 14 pigs were tested using the sequential indirect immunoperoxidase and immunophosphatase methods. Tissues from the tonsils, spleen and cen icai lymph nodes were analysed. It could be seen that the lambda/kappa ratio in dogs, cats, horses and cattle is largely dominated by the lambda chains (lambda/kappa ratio in dogs: 91/9 %, in cats: 92/8 %; in horses: 94/4 %; in cattle: 91/9 %). A more or less balanced ratio was observed only in the case of the pigs (52/48 %).