Complement consumption induced by Fab' fragments of rabbit IgG, irrespectively of their state of aggregation, requires the presence of homologous rabbit IgG. Up to a certain concentration of Fab' fragment, the amount of complement fixed is a linear function of the Fab' fragment dose, but further raising of the Fab'-fragment concentration does not result in complete complement consumption in the sample. The interaction between Fab' fragment and IgG is not strictly species specific.