This new immunodiffusion method involves the incorporation of antigen and complement in a primary agarose gel. Heat-inactivated serum samples are filled into wells and allowed to diffuse radially 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 indicator cells appear. Otherwise, the red bloods cells are lysed. This sensitive technique is employed in a study of human sera for the measurement of the antibody response to the internal antigens (MP and NP) of influenza A virus after infection and vaccination. It was found that none of the vaccinees developed antibodies to the MP antigen, whereas 35% of the post-infection group showed a significant rise. The response to the NP antigen was less frequent and of lower order of magnitude after vaccination than after infection.