A complement fixation technique for the quantitative measurement of antigenic differences between strains of the virus of foot-and-mouth disease

A complement fixation test has been developed to measure the minor antigenic differences which occur between strains of the virus of foot-and-mouth disease of the same immunological type. 2. Strain-specific sera were prepared from each of the virus strains examined and then titrated with each of the antigens. 3. The amounts of complement fixed in each reaction were measured in the region of maximal fixation where the antigen and antibody were present in optimal proportions. 4. Heterologous antigen-antibody reactions were compared with the homologous reaction in each test and expressed as cross-fixation ratios. 5. Pairs of antigens could then be compared by taking the product of their cross-fixation ratios one with the other. The values of cross-fixation product so obtained ranged from 1·0 between identical strains to 0·01 between strains of different immunological type. 6. Strains of virus which exhibit cross-fixation products of 0·5 or more with one another have been classified within subtype groups. I wish to acknowledge the guidance and encouragement of Dr J. B. Brooksby, Director of the Animal Virus Research Institute, Pirbright, and the technical assistance of Mr E. Scoates.