The development of a microtitre fluorescent antibody test, for detection of adenovirus antibody is described. Chick embryo liver cells, infected with fowl adenovirus were found to be more suitable for the test than chick kidney, since a proportion of the chick kidney cells were found to contain non-viral cytoplasmic inclusions which were easily confused with specific viral fluorescence at the low power magnifications used in the test. It was demonstrated that a single positive serum would stain cultures infected with 11 different serotypes of fowl adenovirus, and two serotypes of turkey adenovirus with only minor differences in titre, thus confirming the presence of a common antigen in fowl and turkey adenoviruses. In a survey for adenovirus antibody a total of 595 avian sera (453 fowl and 142 turkey) were screened by double immuno-diffusion and microtitre fluorescence. A total of 52.6% of the sera was positive by double immuno-diffusion while 70.8% were positive by microtitre fluorescence.