Comparison of a 1976 serum survey with one of 1977 has permitted an assessment of the impact of the national swine influenza vaccine program of 1976-1977 on the antibody status of the Michigan population. Prevalence of HI influenza virus antibody in premarital sera collected in 1976 prior to the vaccine program was compared to that in similar sera collected in 1977. Overall prevalence of A/New Jersey antibody (titers greater than or equal to 1:10) in 1976 sera was 22.3%. Little antibody was detected in sera from persons less than 40 years of age and prevalence peaked at age 50. Increased antibody prevalence was found for all age groups in sera collected in 1977 following the vaccine program, and the overall prevalence was 41.6%. Only 3.5% of those under 19 years of age were vaccinated, and post-vaccine prevalence for this group was 10%. This age group, comprising about 30% of the state population, appears to have had least exposure to swine influenza virus, and may be the population segment at greatest risk of infection should strains of this antigenic composition reappear. In contrast, highest prevalence of A/Victoria antibody was found in the 15 to 19 age group, where prevalence was 52%, compared to an overall prevalence at 40%.