of the phorbol triesters are found in this extract, because of their lipophilic nature. 'H-NMR spectroscopy is an interesting method to partially identify and to quantify phorbol esters in mixtures, in semipurified samples, or even in the complete croton oil, and it is possible to determine at the same time the proportion of diesters and triesters. Structural information and quantitative results are obtained simultaneously. This method can be used for the phytochemical screening of cocarcinogenic and cryptic cocarcinogenic phorbol di-and triesters in natural and semisynthetic samples. Because of its speed and accuracy this technique provides a valuable alternative for the less versatile biological methods. The limitation of this physico-chemical method is its restriction to known classes of diterpene esters.