The influence of hot and cold ambient temperatures on cellular immune responses of chickens was assessed, utilizing dinitrofluorobenzene-induced contact sensitivity, phytohemagglutinin (PHA) skin test, and PHA-stimulated, whole blood lymphocyte transformation assays. Dinitrofluorobenzene- and PHA-induced wattle swelling were reduced (P less than 0.05) in birds exposed to air temperatures of 36 degrees C for 1 C for 5 days when compared with wattle swelling in chickens maintained at 26 C. The PHA-induced lymphocyte transformation, as measured by tritiated thymidine uptake, was also suppressed (P less than 0.05) in chickens exposed to either heat or cold stress for 5 days. Some breed differences were observed. These data demonstrated that chronic heat and cold stress impaired expression of contact sensitivity in vivo and proliferation of T lymphocytes in vitro in the avian species. Synthesis of antibodies to sheep RBC on day 5 after antigen injection was not altered (P greater than 0.05) when birds were vaccinated 24 hours before the start of thermal stress. This indicated that B cell and T-helper cell functions were not compromised by chronic thermal stress. However, New Hampshire birds that were heat-stressed had higher (P less than 0.05) antibody titers than control birds 10 days after vaccination.