Granules of blood eosinophils are stained directly by anti-immunoglobulin fluorescein isothiocyanate conjugates.

Direct staining of the granules of blood eosinophils by anti-immunoglobulin fluorescein isothiocyanate (FITC) conjugates was observed when feline blood smears were tested for presence of feline leukemia virus (FeLV) antigen by immunofluorescent antibody. When blood smears of other species including swine, horses, cattle, dogs, sheep, birds, and human beings were examined, direct staining of eosinophils by FITC conjugates was also detected. This FITC staining was restricted to eosinophils and was not observed in neutrophils, lymphocytes, and platelets. Direct FITC staining of eosinophils does not represent a problem in immunofluorescent test for the detection of FeLV infection in cats, as long as the eosinophils, which can easily be recognized as such, are excluded from the spectrum of interpreted cells.