Sera from 9 symptomatic and 7 asymptomatic pigeon breeders were analyzed for their reactivity to pigeon serum by western blotting. All 9 symptomatic sera (9/9; 100%) and only 4 of 7 (57%) asymptomatic sera revealed specific antibodies. The immunoreactivity patterns of the sera varied: the majority of the sera reacted to antigens having molecular weights of 220 kD or more (9/16 sera), 98 kD (8/16 sera), and 86 kKD (8/16 sera). However, only sera from symptomatic breeders recognized an antigen of approximate molecular weight of 29-32 kD (9/9; 100% of symptomatics). We conclude that this antigen is 'disease associated' and may be useful in the diagnosis of pigeon breeder's disease.