AbstractCopaiba oil resin (COR) obtained from Copaifera multijuga Hayne has been used in popular medicine as an antinflammatory and for the treatment of bronchitis, ulcers and cancer. The aim of this study was to evaluate the action of COR and its fractions on the inhibition of lung metastasis and tumour growth induced by B16F10 melanoma cells in mice and cytotoxicity in vitro using Trypan Blue exclusion method and MTT conversion.Mice which have received subcutaneously B16F10 cells developed a solid tumour that reached a peak at 17 days. Together with the increase in tumour growth we also observed an increase in the number of lung nodules. There was a positive correlation between the in vitro cytotoxic assay and in vivo antitumour activity. The oral administration of COR (at 2 g/Kg in the days 3, 5, 7, 10, 12 and 14 after inoculation of tumoral cells) reduced tumour growth by 58% and tumour weight by 76%. At the same dose COR reduced the number of lung nodules by 47.1%. In vitro experiments showed that COR incubated with the melanoma cell line reduced cell viability in a concentration and time‐dependent manner. Diterpenic and sesquiterpenic fractions or reconstituted oil induced cytotoxicity. Our results shows that COR and its fractions have tumouricidal activity in the melanoma cell line in both models in vivo and in vitro. Copyright © 2003 John Wiley & Sons, Ltd.