SummaryLymphocyte reactivity to pigeon antigens was demonstrated in a patient with severe symptomatic pigeon breeder's disease (PBD). The reactivity of the lymphocytes declined subsequent to cessation of exposure to antigen and in parallel with clinical improvement. Lymphocyte reactivity was also demonstrated in another patient with PBD, in a pigeon breeder with no evidence of PBD, and in a laboratory investigator with a history of exposure to birds other than pigeons. Lymphocyte reactivity in the latter subject increased following cutaneous injection of pigeon serum with the development of delayed‐type cutaneous reactivity. Although lymphocyte reactivity to pigeon antigens may be a necessary immunologic feature for production of PBD, this reactivity does not occur only in symptomatic subject