• The incidence of acute respiratory illnesses and the development of neutralizing antibodies are being studied in a comparison between a test group of 4,000 and contratest group of 12,000 naval recruits. Each member of the test group received a single 2-ml. intramuscular injection of a commercially prepared formaldehyde-inactivated APC (adenoidalpharyngeal-conjunctival virus) vaccine, types 3, 4, and 7. The dosage was fixed by preliminary studies in which the vaccine was compared with a placebo, the cause of pain after injection was discovered and obviated, methods of study were tested, and the proportion of recruits to be vaccinated was decided. Laboratory data showed that the vaccine induced a substantial neutralizing antibody response to types 3, 4, and 7 adenovirus. The clinical data of the progress report, which covers only one-third of the experience, showed that the vaccine effected a substantial reduction, estimated at 50 to 75%, of the total expected febrile respiratory illnesses, which in this study were associated mainly with type 4 adenovirus.