Journal of Biological Standardization (1985) 13, 15 I-I57 The titration of tetanus antitoxin V. Effect of formalization method for the fixation of sheep erythrocytes on the indirect haemagglutination test* R. K. Gupta, t S. C. Maheshwarit and H. Singht Two methods of fixation of sheep erythrocytes with formaldehyde for the titration of tetanus antitoxin by the indirect haemagglutination (IHA) test have been compared. The cells fixed with 3% formaldehyde at 4-8°C for 24 h (formaldehyde (I) fixed cells) were less sensitive than the cells fixed with 3~ formaldehyde at 4-8°C for 24 h and subsequently treated with 409~ formaldehyde at 4--8°C for a further 24 h (formaldehyde (1I) fixed cells). The correlation between the toxin neutralization (TIM) and IHA titres using formaldehyde (I) fixed cells was better than that obtained with formaldehyde (II) fixed cells. There was no statistically significant difference between TN and IHA titres after treatment of the sera with 2-Mercapto- ethanol using formaldehyde (I) fixed cells. Formaldehyde (I) fixed cells can be used for two months with adequate sensitivity to detect the minimum protective level of tetanus antitoxin in the sera. INTRODUCTION The indirect haemagglutination (IHA) test has been widely used throughout the world for detecting and quantitating tetanus antibodies in human and animal sera. Mostly fixed erythrocytes are used in preference to unfixed cells for this test. It has been reported that erythrocytes preserved with g ataraldehyde or formaldehyde show altered sensitivity in comparison with fresh, unfixed cells, resulting in great discrepan- cies from the titres obtained with the toxin neutralization (TN) test. * Lavergne et al. 2 found that fresh red cells gave IHA reactions which differed from those obtained with o Received for publication 9 July 1984. t Division of Biological Standardization & Quality Control. Central Research Institute, Kasauli (Himachal Pradesh), India. 0092-- 1157185/020151 +07 $03.00/0 ~)!985 The International Association of Biological Standardization 151