Samples of serum from 970 people in Nova Scotia, Canada, were tested by radioimmunoassay for determination of the prevalence of antibodies to hepatitis A antigen (anti-HA). Volunteer blood donors aged 16--26 years or 51 years old or older provided 575 samples. Other samples were obtained from 152 students accepted for nursing training at the Victoria General Hospital, Halifax, Nova Scotia, and from 243 patients of the Special Urology (sexually transmitted diseases) clinic of the hospital. Percentages of samples positive for anti-HA were determined according to classification of the donors by age, rural vs. urban location, sex, and attendance at the clinic. Samples were positive for 43.4% of the male patients and 45.8% of the female patients at the clinic, 13.8% of the student nurses, 12.6% of the young blood donors, and 69.1% of the older blood donors. A higher prevalence of anti-HA in donors was associated with increased age, urban environment, and attendance at a clinic for treatment of sexually transmitted diseases.