1. By means of microsphectrophotometry the author carried out quantitative analyses of DNA and protein in an individual cell, using Euplotes woodruffi and Ehrlich ascites tumor cells as the materials through Feulgen reaction and Naphthol yellow S stain, and studied the relation between the quantities of these substances and the mitotic cycle. The results are as follows. 2. The DNA content in the macronucleus of Euplotes is increased at two ends of the nucleus, and the part where DNA is increased and that which shows no DNA increase are demarcated by the formation of a reorganization band. 3. The DNA synthesis of Euplotes is most active in the latter stage of interphase and the DNA content is completely doubled just before the cell division. 4. The protein synthesis of Euplotes cells is taking place at all phases of the mitotic cycle and there is a linear relation between the mitotic cycle and the protein contents of the cell. 5. Judging from the relationship between the DNA synthesis and the protein synthesis during the mitotic cycle of Euplotes cells, the DNA content and the protein content in an individual cell show a sigmoid curve between the two and also the similar curve can be observed in the case of Ehrlich ascites tumor cell. Therefore, it is assumed the protein synthesis is being likewise carried on all through the mitotic cycle of Ehrlich ascitess tumor cells. 6. The author discussed about the relationship between the protein synthesis and the cell division.