The advantages, disadvantages, and uses of serological epidemiology are discussed in relation to acute respiratory infections in military personnel. The prevalence of antibody reflects both current and past experience with respiratory agents and is a measure of susceptinility. Incidence data calculated by testing two serial serum samples, on entry and discharge from the service, has indicated high influenza and mycoplasma pneumoniae rates in South American recruits and low rates of adenovirus and parainfluenza infections. Serologic analysis of reinfection rates showed high protection against influenza infections at HI antibody levels of over 1:40, against adenovirus infections at neutralizing titers of 1:5, and against M. pneumoniae infections at TRI antibody levels over 1:8. Antibody responses persisting at least 7 mo following immunization were demonstrated in 70% of 428 vaccinated young adults for A2 antigen and 20% for influenza B antigen. No relation of ABO blood groups to respiratory infection was found. The lack of myxovirus infections in four Polaris submarines is presented.