Human chromosome preparations were subjected to a wide range of treatments — heat, enzymatic digestion, acid, and alkali. These treatments included the conditions of a standard Giemsa-banding procedure, a trypsin-banding method, and the reverse-banding procedure of Dutrillaux and Lejeune (1971). The slides were then stained with acridine orange, which fluoresces green in combination with double-stranded, and red with single-stranded, nucleic acids. Under various conditions, including those of the three banding procedures mentioned, chromosome banding was obtained with acridine orange. The pattern was always the same and resembled “reverse” banding, with bright yellow/green fluorescence in the regions of R-bands. Quinacrine and standard Giemsa-bands corresponded to dull red/orange staining with acridine orange. The results are interpreted as suggesting that chromosome banding patterns correspond to regions whose DNA differs in the ease with which it denatures and the extent to which it reanneals. These properties could be related to variation in associated chromosomal pro