Aims: The study was conducted to evaluate the possibility of selecting convalescent diphtheria patients to serve in emergency situations as donors for the production of anti-diphtheria immunoglobulin. To select suitable donors, the criterion of an antitoxin titer ≥3.0 IU/ml was used. In addition, the effects of treatment and the effect of immunization with diphtheria toxoid on the level of anti-diphtheria toxin antibodies were evaluated. Scope: Three groups of diphtheria patients were included in the study. The first group (n=23) consisted of patients who had a basic antibiotic treatment, with or without serotherapy using horse antitoxin and/or human immunoglobulin. The second group (n=12) comprised patients examined immediately after the onset of disease. The immunological history of this group was not known. The third group (n=20) included patients with a known immunization history, treated only with antibiotics but having received a booster immunization with diphtheria toxoid. Antitoxin titers were measured using the toxin binding inhibition (ToBI) assay. Conclusions: In the first group, 47.8% (11/23) of the patients had a diphtheria antibody titer ≥3.0 IU/ml. For most of them, however, the antibody titers could have resulted from treatment with exogenous antibodies from horse antitoxin or human immunoglobulin (18/23). Only two of the 11 high-titer subjects had received antibiotics only. Among the second group, only two (16.76%) of the patients had an antibody titer of ≥3.0 IU/ml. In the third group 50% (10/20) of the patients showed an antibody titer of ≥3.0 IU/ml prior to vaccination, and therefore could be directly considered as donors. Three weeks after booster vaccination, 70% (14/20) had an antibody titer of ≥3.0 IU/ml and 1 year after booster vaccination, 28.6% (2/7) of the subjects still had titers of ≥3.0 IU/ml. In 40% of these patients, a decrease was observed 3-4 weeks after the booster dose. It was concluded that convalescent diphtheria patients could be considered as donors in an emergency situation, since approximately half of them showed antitoxin titers of ≥3.0 IU/ml. © 2003 Elsevier Ltd. All rights reserved.