The degree of seroprotection against diphtheria in Poland was evaluated by determination of IgG antibodies to Corynebacterium diphtheriae toxin (IgG-DTAb). The study population consisted of 4829 healthy subjects aged from 1 day to 85 years from 7 regions of Poland. Serum samples collected between 1996 and 1998 were assayed for IgG-DTAb antibodies using a toxoid enzyme immunoassay. Neutralization of toxin in Vero cells was performed as a reference method with the WHO standard for human diphtheria antitoxin. The study revealed a lack of seroprotection (IgG-DTAb < 0·1 IU/ml) in 23 % of individuals, basic seroprotection (0·1–1·0 IU/ml) in 64 %, and effective seroprotection (> 1·0 IU/ml) in 13 %. The non-protected group consisted of non-vaccinated children below 2 months of age (10 %), individuals between 2 months and 18 years old (20 %) and greater than 19 years old (70 %). Of the adults, 32 % were seronegative, 63 % had basic seroprotection and only 5 % were fully protected; 43 % of adults between 30 and 64 years, who had not been vaccinated at least during the previous 10 years were not protected against diphtheria. The geometric mean titre (GMT) of IgG-DTAb was 0·25 IU/ml in the total population. Age-related GMTs differed significantly from each other and were higher (0·44 IU/ml) in individuals from 2 months to 18 years old, compared with 0·14 IU/ml and 0·17 IU/ml in children under 2 months and adults, respectively. No significant difference was found in the GMTs of men and women in all age groups. We conclude that the currently used vaccination programme in Poland is highly effective and assures protection against diphtheria in the majority of the population in the 10-year period following the last booster. However, a significant proportion of adults between 30 and 64 years lack protection and this indicates a need for booster immunization for this group.