In the course of searching for bioactive compounds from Croton species from Venezuela, two seco-entkaurenes isolated from flowers of Croton caracasana were evaluated in vitro for their effect on cell viability by the standard MTT assay in nine human cancer cell lines of different origins and one primary culture. Both compounds induced cytotoxicity in the range of 2 to 25 μM for caracasine and 0.8 to 12 μM for caracasine acid. However, for the normal fibroblasts and the cell lines, HeLa, MCF-7, PC-3, LoVo, X-17, Jurkat E6.1 and Jurkat JCaM1.6, the IC₅₀ values of caracasine acid were lower than their counterparts. Interestingly, no differences in IC₅₀ were recorded for the leukemic cell lines U937 and K562. It can be concluded that the acid moiety in the structure enhances the cytotoxic effect of caracasine by a pathway which seems not to be activated in the leukemic cell lines tested.