Although adneovirus type 19 (Ad19) was first described in 1955, this virus was not associated with disease until its isolation from outbreaks of conjunctivitis in 1973. A strain of Ad19 isolated from a case of conjunctivitis in Seattle in 1974 was compared with the reference strain (3911). Plaque number and size were enhanced by 30 mM MgCl2. Low pH and chloroform treatment had no effect on either strain's activity, but the two strains were sensitive to pH 8. Growth curves were characteristic of adenoviruses, but differences were seen in the amount of virus released. The ratios of particles to plaque-forming units (approximately 10,000:1) were similar for both. Both virus preparations contained high concentrations of group-specific complement-fixing antigen. Cross-reactions were seen by hemagglutination inhibition and immunoelectron microscopy between antisera to Ad8, Ad9, and Ad10 versus both strains of Ad19, but were not seen by neutralization. We would like to suggest, based on exclusive conjunctivitis association and cross-reactions, that the four cross-reacting serotypes, Ad8, Ad9, Ad10, and Ad19, represent a subgroup of adenoviruses specfically associated with conjunctivitis.