A series of novel betulinic acid derivatives 3-11 and betulin derivatives 12-17 were synthesized. The compounds were characterized by the means of (1)H- and (13)C-NMR spectroscopy as well as mass spectrometry. The compounds have been tested on ten tumor cell lines of different histogenic origin. The most active derivatives, containing a chloroacetyl group on C-3 in betulinic acid 9 and C-28 in betulin 15, were up to ten times more cytotoxic and many fold more selective towards tumor cells in comparison to normal cells (fibroblasts) than betulinic acid. Furthermore, compound 15 was found to possess cell growth inhibition even when treated for a short time on anaplastic thyroid cancer cells (SW1736).