The tests for tetanus aetiological diagnosis are briefly surveyed: with particular attention to Passive Haemagglutination on Turkey Red Blood Cells (P.H./T.R.B.C.), for the assessment of antitoxin serum level. The P.H./T.R.B.C. proves a valuable tool in different occasions: -in patients, to control the immunotherapy; -in subjects at risk, to apply a correct prophylaxis; -in the whole population, to verify the outcome of compulsory vaccination.