Evidence is presented to show that bismuth combines in vitro with the phosphate of nucleic acids in a manner similar to its reaction with inorganic phosphate. When tested under similar conditions, protein exhibited no attraction for bismuth. The results of the in vitro experiments, which are of interest within themselves, may be indirectly applicable to in vivo staining. Dividing cells of onion root tips were fixed in OsO4, stained with bismuth, and examined in the electron microscope. The electron opacity of cell structures known to contain nucleic acids was enhanced by bismuth, while organelles known to lack appreciable quantities of DNA or RNA showed little, if any, change. Bismuth is particularly effective as a stain for the chromatin material during interphase and for the chromosomes during division.

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