Alanine aminotransferase (AAT) is mainly synthesized in the liver, and its level in mammalian serum is elevated after acute phase induction. Here we demonstrated that sheep anti-human AAT antibody cross-reacted with amphioxus humoral fluids as well as human serum; and the concentration of AAT in the humoral fluids in amphioxus increased after the acute challenge with lipopolysaccharide, while the level of total proteins remains unchanged. These suggest the presence of the same acute phase response pattern in amphioxus, as observed in some mammalian species. Immunohistochemically, AAT was localized in the hepatic diverticulum, ovary and testis. It appears that the hepatic diverticulum in amphioxus is functionally homologous to the vertebrate liver in respect of AAT synthesis, supporting the hypothesis that the vertebrate liver evolved from the hepatic diverticulum of an amphioxus-like ancestor during early chordate evolution.