Although modern medicine made great steps toward curing most diseases considered deadliest, yet cancer remains one of the major public health problems. The first part of this thesis investigates a new source of selective compounds that are potential candidates against cancer. Fifteen Russian compounds were tested in order to establish their efficiency against two types of cancer: human breast SK-Br-3 and colorectal carcinoma HT-29. The bioassay results show that seven of the new synthetic Russian compounds can be considered new drug leads, based upon their low toxicity and efficacy in slowing the growth of human breast cancer and colon cancer cells. The goal of the second part of the thesis was to isolate pure compounds that inhibit the growth of cancer cells from the methylene chloride extract of Eucalyptus viminalis Labill. This plant was selected for investigations since a preliminary screening of plants from Russia indicated that had activity against cancer. The result of this work was the isolation of a pure compound which has been analyzed using different spectroscopic techniques such as MS, HPLC, 1H-NMR, DEPT, 13C-NMR. The extracted compound was â-amyrin formate, which was previously reported by Malhotra,[49] from Canarium strictum Gum in 1987. Also, the bioassay results indicated that â-amyrin formate might be considered a possible drug lead against colon cancer and can be recommended for further investigations. This is the first report of isolation of â-amyrin formate from Eucalyptus viminalis Labill and the first test of the activity of this compound against colon cancer.