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Single radial immunodiffusion (SRID) analysis of reference antigens in the presence of homologous and heterologous potency antisera. Dilutions of four H5N1 potency reference antigens were analyzed by SRID using potency antiserum for A/Vietnam/1203/2004 (A), A/Indonesia/5/2005 (B), or A/bhg/QL/1A/2005 (C). Precipitin rings were measured in two directions to the nearest 0·1 mm for the determination of diameter.
