Visualization of banded polytene chromosomes under epifluorescent illumination by counterstaining with ethidium bromide.

Current methods of localization of chromosomal antigens on polytene chromosomes of Drosophila melanogaster salivary glands by indirect immunofluorescence require comparison of two microscopic images of the same nucleus--a phase contrast or bright field image to visualize the chromosomes and a fluorescent image to locate the antibody. We have found that inclusion of the DNA-intercalating agent ethidium bromide in the mounting medium makes banded polytene chromosomes visible under epifluorescent illumination, eliminating the need for two images. The banding of the polytene chromosomes is clear enough to locate specific bands.