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The validity of the ammonium sulphate precipitation technique of estimation of antibody amount and avidity.

Determinations of antibody titres with the ammonium sulphate precipitation technique showed an accuracy of ± 10 per cent (P<0.05). Calculation of antibody avidities with Sips' distribution formula, using suitably diluted antisera, gave a ten times greater accuracy than either undiluted or too highly diluted antisera. The accuracy was lower with the formula of Celada, Schmidt and Strom (1969) probably due to the narrow antigen ranges employed. Comparison of avidity calculation according to Sips (1948) and Celada et al. (1969) showed a good correlation (r = 0.73–0.79; P<0.01). No correlation between antibody titre and antibody avidity values in the sera (r = 0.03–0.19) was observed.