The effect of tricyano-2-amino-l-propene and Na-nitroferricyanide were studied on growing of Ehrlich ascites carcinoma, Voshida ascites sarcoma, leukaemia LY 1210 and NK Ly of mice, and Shay myeloid chloroleukaemia of rats. TAP partly prolonged the life of Ehrlich carcinoma. This substance in combination with cyclophosphamide or mielobromol had the opposite effect, i. e. it forced growing of the cancer. NNP had a better effect on Ehrlich ascites then TAP. TAP had no effect on Yoshida sarcoma, while NNP had. Neither TAP nor NNP diminished the number of leukocytes administered daily for 28 days. TAP was completely without effect on both kind on lymphatic leukaemia’s, and on chloroleukaemia. NNP prolonged both lymphatic leukaemia’s, but had no effect on myeloid leukaemia. It has been discussed about these findings, because both of these substances are chemically related (cyan groups). One of them (TAP) forced the synthesis of nuclein acids. It is also important that these two substances are being used in human therapy, but for the other purposes. Preliminary clinical results with NNP showed a certain therapeutic effect in children suffering from leukaemia.