This paper deals with the conformational study in solution of leptocarpin, a germacranolide with the heliangolide structure isolated from Leptocarpha rivularis D.C. The results obtained agree with the single crystal X-ray diffraction studies. Leptocarpin and leptocarpin acetate show 50% inhibition of [35S]-methionine uptake without affecting the (methyl-3H)thymidine and (5,6-3H)uridine incorporation, when assayed in HeLa cells.