A radial immunodiffusion assay for serum antitetanus antibody has been in routine use in this laboratory for 12 months. It has proved simple and reliable for the screening of normal blood donors. Earlier preliminary observations - that (a) 4.4% East Anglian donors have antibody levels of 5 IU/ml or more and (b) 29% of such donors maintain more or less constant antibody levels for a considerable time without further known stimulation - have been confirmed.