The cell-mediated immune response to pigeon antigens was assessed in 12 symptomatic pigeon breeders, 14 asymptomatic breeders with precipitins, 25 asymptomatic breeders without precipitins and 10 control subjects. Significant lymphocyte stimulation by pigeon serum and/or pigeon droppings extract was obtained in 11 of the 12 symptomatic breeders but was not obtained in any of the other subjects studied. The lymphocyte response to pigeon antigens in culture is presented as the distinguishing criterion for the differentiation of symptomatic and asymptomatic pigeon breeders. This study supports a direct role for a T cell-mediated immune response to pigeon antigens in the immunopathology of pigeon-breeder’s disease.