Abstract: We reported previously the antitumour activity of Sb-derivatives of complexones. It was demonstrated that Sb-71 (Sb ammonia triacetic acid) might significantly retard the growth of Ehrlich ascites carcinoma and spindle cell sarcoma in mice, and Guerin carcinoma in rats. It will be of interest to study the direct action of the drug on the tumour cells. In this work, smears of ascitic fluid of mice 5—6 days after inoculation of Ehrlich carcinoma were stained by means of Feulgen reaction. The morphologic changes of carcinoma cells were examined before and after Sb-71 treatment. Experiments were performed on 49 mice bearing Ehrlich ascites carcinoma. A single intraperitoneal injection of 30mg/kg of Sb-71 caused a considerable drop in the mitotic index (counted 1000 cells) within 24 hours, and the maximum action was attained in 6—12 hours after injection. The drug depressed all four stages of mitosis, and the interphase cells showed also definite changes, such as the conglutination of the structure of nuclear chromatin, increasing of degenerated cells which occurred in 1—12 hours after injection. The same dose of Sb-71 administered daily for 7 days produced more marked inhibitory effect on Ehrlich carcinoma cells. In the smears of ascitic fluid, dividing cells disappeared and a great portion of cells were contracted in size. Their nuclei exhibited a marked, agglutination and karyorrhexis with a deep stain. In the meantime, the concentration and the total number of ascites carcinoma cells were also significantly decreased in comparison with the control group. The intravenous injection of 30mg/kg of Sb-71 caused only a slight effect on mitotic activity in 1—3 hours. From the above-mentioned data, it seems that the inhibiting action of Sb-71 upon Ehrlich carcinoma is mainly due to its direct effect interfering with the mitotic process of carcinoma cells.