The ability of the sodium salt of 3,4-secoisopimar-4(18),7,15-trien-3-oic acid (1), a diterpenoid obtained from Salvia cinnabarina, to inhibit the genotoxic effect of ethyl methanesulfonate (a clastogenic agent) and colcemid (an aneugenic agent), was studied using a micronucleus assay on cultured human lymphocytes. Cells were treated with 1 before (pretreatment), during (co-treatment), and after (post-treatment) treatment with the mutagens, in order to investigate the type of antimutagenic activity (desmutagenic or bioantimutagenic) manifested. In the range of concentrations tested (0.3-330 μM) 1 reduced significantly the frequency of micronuclei induced by ethyl methanesulfonate, in both pre- and co-treatment protocols (up to 74% and 70% of reduction, respectively), showing an anticlastogenic activity. Conversely, 1 did not inhibit the effect of colcemid in all treatments. The nuclear division index value of lymphocytes was not affected by treatment with 1, thus demonstrating that the anticlastogenic effect of 1 was not due to a cytotoxic effect. On the basis of the results obtained, it can be hypothesized that 1 exerts its anticlastogenic activity against ethyl methanesulfonate by a desmutagenic mechanism, possibly by chemical inactivation of the mutagen.