Chromosome measurements of length and area, performed with the help of PIQUANT, our computer system, have been combined with the quinacrine-mustard fluorescent-banding technique to characterize the karyotype of individuals. Five sets of monozygotic twins were examined, and it was found that (1) individuals differ significantly in their chromosome measurements, particularly in the size, and (2) the within-twin pair differences are very small and not significant, thus establishing the genetic basis of the differences under (1). The observed differences are not only statistically significant but large enough to permit selection of all five sets of twins from only the chromosome measurements of their autosomes. The potential of these measurements to serve as genetic markers is shown to be considerable, whenever 10 to 30 cells per person, all banded, can be measured.