Humoral parameters of amphioxus Branchiostoma belcheri, including lysozyme, antimicrobial activity, microbial agglutinin and haemagglutinins were measured before and after challenge with Escherichia coli. Humoral fluids from unchallenged B. belcheri had lysozyme, antimicrobial, microbial agglutinating and haemagglutinating activities, which may represent part of the baseline level of innate immunity in this organism. After challenge with E. coli, the lysozyme activity, growth-inhibiting activities against E. coli and Vibrio alginolyticus, microbial agglutinating activities against Micrococcus lysodeikticus, Bacillus subtilis and Staphylococus aureus, and haemagglutinating activities against rabbit and human A and O erythrocytes in the humoral fluids were all increased significantly. In contrast, the agglutinating activities against Vibrio harveyi and E. coli and the haemagglutinating activity against human B erythrocytes in the humoral fluids were reduced in response to E. coli challenge. It appears that the humoral fluids of B. belcheri contain components that are able to differentiate different microbes and different human blood cell types.