The prevalence of Arkansas (Ark)-type infectious bronchitis virus (IBV) in Delmarva peninsula broiler-type chickens was determined. The immunity of 5-to-11-week-old commercial broilers was evaluated by intraocular inoculation with Ark-type DPI strain (Ark DPI) challenge virus and collection of tracheal swabbings 5 days later. Serum Ark-type antibody titers were obtained using the virus-neutralization test. Eighty-five flocks were tested from January to August 1981. Nearly 60% of the flocks had substantial (greater than or equal to 70%) local immunity of the upper respiratory tract. Twenty-two percent had intermediate (50-69%) and 19% of the flocks had low (less than or equal to 40%) levels of local immunity. Serum antibody titers generally agreed with challenge results. In addition, high Ark-type IBV neutralizing-antibody titers were found in 16 Delmarva broiler breeder flocks. Seven current IBV field isolates were characterized for antigenic similarity to Ark DPI. Four isolates contained Ark antigen(s) based on significant neutralization in virus-neutralization tests and on substantial immunity to challenge afforded by Ark DPI virus immunization. Three isolates did not appear to contain Ark antigen(s). Immunization of chickens with Ark DPI virus afforded substantial protection against Connecticut- and homologous-type virus challenge, partial immunity (63%) against JMK, and no protection against the Massachusetts 41 strain of IBV.