After transplanting Yoshida sarcoma cells under the skin of the thigh of hybrid mice, the author injected 0.2cc of 2% unsaturated fatty acid fraction (denominated as OX substance) intramuscularly and observed the changes in the histological picture, in the mitotic phase as well as the changes in the quantity of DNA per cell, along with the lapse of time, and obtained the following results. 1. The histological pictures revealed the phenomena such as an increase in necrotic foci, a general diminution in stainability and indistinct cell boundary. 2. In counting the number of those cells with a relatively strong activity in cancer foci at various mitotic phases, it has been found that from 6 to 12 hours after the OX injection the number of the cells in metaphase is decreased and from 24 hours on the number of such cells approaches the normal. 3. In the estimation of DNA content per cell, from 6 to 12 hours after the OX injection the number of the cells undergoing a relatively active proliferation but with a decreased DNA content tends to increase but after 24 hours the content per cell approaches the normal level.