Sera from patients with pigeon breeder's disease were analysed for precipitating antibodies by immunodiffusion and immunoelectrophoresis using whole pigeon dropping extract (PDE) and a purified fraction of PDE (PDE1) as antigens. For comparison, sera from asymptomatic pigeon breeders and normal individuals were also tested for precipitating antibodies. Whereas whole PDE formed precipitin lines with normal serum as well as with serum from symptomatic and asymptomatic pigeon breeders, PDE1 formed precipitin lines only with serum from individuals exposed to pigeons, This suggests that whole PDE forms non-specific (non-antigen-antibody reactions) as well as specific precipitin lines, while PDE1 appears to form only specific lines. These data indicate that whole PDE has limited usefulness in studies of pigeon breeder's disease. It is also possible that PDE1 will be useful in studies of this disease.