Serum samples from 2,091 persons from Illinois populations of swine abattoir employees, 4-H Club youths and their advisors, veterinary medical students, pork producers, and veterinarians were tested for hemagglutination-inhibition (HI) antibodies to A/Swine/Ill./63. A total of 231 positive reactions of 1:20 or above were found (11%). The highest prevalence of antibody was found in veterinarians, followed by pork producers and future swine abattoir employees. Age-adjusted reactor rates were highest in pork producers. Of 305 abattoir employees from whom two serum samples were examined 26(8.5%) had positive seroconversions. The abattoir population studied consisted largely of males under 40 years of age. A low prevalence of antibody was found in veterinary medical students and none was found in 4-H Club youths having swine contact. It is postulated that in Illinois there is occupational exposure of veterinarians, pork producers, and swine abattoir employees.