The newly recognized APC group of viruses consists of at least 14 serologically different viruses each sharing a common complement fixing antigen and having common biological properties. Many of these viruses are highly prevalent in the human population, and representatives also occur in certain animals. Types 1, 2, and 5 have been found most frequently in association with febrile respiratory infections in young children, and are unmasked in tissue cultures of adenoids and tonsils of most children undergoing tonsillectomy-adenoidectomy. Type 3 occurs in both civilian and military populations, where it causes a distinct disease entity, pharyngoconjunctival fever, sporadic cases of febrile pharyngitis and follicular conjunctivitis, as well as ARD. Types 4 and 7 cause a significant proportion of ARD and atypical pneumonia in military recruits. Types 6 and 10 have been isolated infrequently, and in association with conjunctivitis. There is evidence that type 8 may be the cause of epidemic kerato-conjunctivitis.