A new immunodiffusion technique in agarose gel for the quantification of complement-fixing antibodies is described. The test involves the incorporation of antigen and complement in a primary agarose gel. Heat-inactivated serum samples are allowed to diffuse radially from wells overnight at 4 degrees C. A secondary gel, containing antibody-coated sheep erythrocytes, is layered on top of the first gel and the system is incubated for 45 min at 37 degrees C. Where complement is fixed, i.e., around wells with positive serum samples, zones of unlysed cells appear. There is a straight line relationship between zone areas so produced and log(2) serum titres obtained with the conventional complement fixation test. The method appears to be applicable to a variety of antigens. It has been found suitable for bacterial and viral antigens. The test can also be reversed, thus allowing the quantification of diffusible antigens in a gel containing immune serum and complement. This paper describes in detail the use of this method as a diagnostic tool for the assay of complement-fixing antibodies to the type-specific antigens of influenza virus in paired human sera.