Complement activation properties of ricin holotoxin, its A or B chain were assessed in two enzyme immunoassays (EIA). One was specific for C3 activation products and the other detected the terminal SC5b-9 complement complex (TCC) and thus determined activation of the initial and terminal part of the complement pathways, respectively. Ricin and its A and B chains were incubated with normal human serum or EDTA-serum. It was found that ricin B chain activated both the initial and terminal complement pathways in a time- and concentration-dependent fashion, whereas the A chain and the holotoxin did not.