To study the magnitute of antitoxic immunity against diphtheria in different age groups of Rostov-on-Don city and Rostov region population.Antitoxic immunity against diphtheriawas studied in children 3 - 15 years old (661 persons) and adults 16 - 59 y.o. (893 persons), which were tested during 2003 - 2007 using direct hemagglutination assay manufactured by Mechnikov Scientific Production Association "Biomed" (Moscow).Proportion of children 3 - 15 y.o. seropositive to diphtheria was 98.6 +/- 0.5%. Children aged 3 years and 4 - 6 years were identified as a risk group for diphtheria as they had highest proportions of non-immune persons. Proportion of immune to diphtheria in adults was 94.4 +/- 0.8%. Maximal level of protection was observed in personsaged 16 - 17and 18 - 19 years, in which the proportion of seropositive persons was 99.1 - 100%. Persons > or = 50 years old formed group of risk because persons with low or moderate titers of antitoxin were identified in this age group mostly.Obtained results provide evidence on enough high level of protection of Rostov-on-Don cityand Rostov region population against diphtheria. Group of increased risk comprised of persons aged 3 - 6 years and > or = 50 years.