Abstract
The deoxyribonucleic acid (DNA) content of individual human leukocytes was estimated cytophotometrically using visible light and spreads stained either with gallocyanin-chrome alum following ribonuclease digestion or with the Feulgen reaction. When the cells were measured on a scanning cytophotometer, significant differences in stain intensity were found among slides. Significant differences also were found among the leukocyte types. In gallocyanin-chrome alum preparations, monocytes measured 16% higher than small lymphocytes and 13% higher than neutrophilic granulocytes. In Feulgen preparations, monocytes measured 4% higher than small lymphocytes and 6% higher than neutrophils. These differences among cell types were independent of donor and stain intensity. Measurements of cells within types and within slides frequently showed close agreement, but it is only in this very limited context that the data are consistent with the hypothesis of DNA constancy. Measurements made on a two-wavelength cytophotometer showed a divergence of only 2.1% relative to similar measurements made on the scanning cytophotometer, which suggests that the differences observed among cells and types are unlikely to be artifacts of the instruments. Over-all, the data indicate either that there is variability in DNA content or that DNA is not being expressed correctly by the measured stain content.