With the purpose to pursue the host-parasite relationship in leprous cells the author studied microspectrophotometrically how the desoxyribonucleic acid synthesis takes place and the results of the study are presented herewith. In the Lepra nervosa the DNA content of the cyrindrical cells in the skin, peripheral lymphocytes, and leucocytes in other tissues conteain the identical qutntity of DNA as in the lymphocytes in the spleen of Lepra tuberosa there is no marked difference from that of normal persons, but along with the proliferation of the bacilli in the liver of same Lepra tuberosa the nuclear volume as well as the DNA content increase slightly. However, in the liver interstitial cells a slight increase or a marked decrease in the DNA contents can be observed. In the nodes of the skin can often be seen cells containing the volume twice the amount of normal DNA content or one half the normal content, demonstrating an inbalance between the DNA synthesis and divisions of cells or nuclei. The cells losing DNA, however, take sooner or later a regressive degeneration along with the destruction of nuclei. Finally the morphology and the function of cells in the precesses of vacuolation and the changes in DNA and RNA are discussed.