A biometrical study carried out on 50 sera, concerning relationships between ELISA 31-13 passive hemagglutination (PHA), both previously standardized against the neutralization test (NT) for the evaluation of antitetanic imunity level, is described, The statistical analysis of intermethods regression and of variance allowed for the estimation of the immunological proteation limit established at a 0.3 IU/ml value (upper confidence limit, p ⩽0.05) in terns of PHA, corresponding to 0.015 IU/ml in terms of ELISA at the same probability level, The correlation coefficient was 0.94, The precision of ELISA was significantly higher in comparison with those of NT and PHA The reasons for a selective choice of the proper type of test in various circumstances are discussed.