Previous measurements of the composition of metaphase chromosomes yield values for the DNA mass fraction ranging from 15 - 32% (eg. 1), with a factor of two variation seen for chromosomes of a single species. The factors responsible for this variation are not known, but understanding them is of fundamental importance for constructing a realistic model of metaphase chromosome structure. The Scanning Transmission X-ray Microscope (STXM) provides the ability to make quantitative measurements of unstained, unfixed, thick wet specimens with a resolution of 55 nm. We have used the STXM to measure the total mass, density, size and shape of fixed wet chromosomes from three plant species and one marsupial species with differing amounts of DNA per chromosome. These measurements allow us to determine the dependence of chromosome composition on the amount of DNA per chromosome, and to determine whether chromosome composition differs between the plant and animal kingdoms.Metaphase chromosomes from the broad bean Vicia faba had a measured DNA mass fraction of 0.38. This value was obtained from both wet and freezedried specimens although the size and shape of the chromosomes resulting from these different treatments varied considerably (Fig. 1). This result indicates that the DNA mass fraction value obtained is not a property of the preparation procedure. Because dry specimens are not subject to mass loss at these exposure levels, the mass results for both the wet and dry specimens is not an artifact caused by radiation damage.