The butanolic extract and the isolated chemical constituents, mainly diterpenes and flavonoids, from Greek propolis have been tested for their cytostatic activities against human malignant and normal cell strains. The extract and the diterpenes were found to be the most active against HT-29 human colon adenocarcinoma cells, without affecting normal human cells. Manool, a diterpene isolated for the first time from Greek propolis, was the most active compound, arresting the cancer cells at the $G_2$/M phase of the cell cycle.