Large outbreaks of diphtheria occurred recently in the former USSR. Between 1989 and 1994, a total of about 600,000 Soviet immigrants arrived in Israel. The immune status against diphtheria in a sample of 992 men aged 17-49 and 195 women aged 17-19, who arrived in Israel during 1990-91, was studied in order to evaluate the need for vaccination. Participants completed a self-administered questionnaire and diphtheria antitoxin antibody levels were measured by means of ELISA. At age 17-19, the prevalence of antitoxin antibody levels below the protective level of 0.01 IU/ml was 4.8% in the men and 2.1% in the women. Among the men, the percentage lacking protection declined from 4.8% at age 17-19 years to 1.6% at age 20-24, and increased to 18.2% at age 35-49. In the oldest group, the prevalence of those lacking protection was considerably higher than for the general Israeli population. In the multivariate analysis, age, mother's education and republic of origin were significantly associated with the absence of protection. Immigrants from the former USSR appear to be more susceptible to diphtheria, thus increasing the possibility of clinical disease, and it is recommended that they receive booster doses of diphtheria toxoid.