Circulating autoantibodies to mesothelin were detected in sera of hens with mesothelin mRNA expressing ovarian tumors by immunoassay. Sera were tested at dilutions of 1:100 and 1:200. Four of the nine hen sera with mesothelin PCR positive ovarian tumor are positive at both dilutions. None of the sera from hens without mesothelin mRNA expression had an optical density (OD) above the cutoff value for mesothelin antibody. The cutoff was determined as the mean value of normal controls plus two standard deviations above the mean. The cutoff value for positive reactions was 0.49 at 1:100 and 0.23 at 1:200.