The cytotoxic effects of the essential oils obtained from the flowering aerial parts (APO) and ripe fruits (RFO) of Echinophora spinosa L. (Apiaceae) from central Italy toward human U937 promonocytoid cells were studied; the contribution of each of the major constituents to the whole cytotoxic activity of either APO or RFO was also characterized. The major components of APO were beta-phellandrene (34.7%), myristicin (16.5%), p-cymene (16.3%), delta3-carene (12.6%), alpha-pinene (6.7%) and alpha-phellandrene (6.2%); those of RFO p-cymene (50.2%), myristicin (15.3%), alpha-pinene (15.1%) and alpha-phellandrene (8.1%). Both oils tested were toxic to U937 cells, but RFO was much more cytotoxic: indeed, the IC50 values calculated from the linear regression curves of RFO and APO were 14.5 +/- 0.85 and 43.4 +/- 2.81 microg/mL, respectively. alpha-Pinene and alpha-phellandrene were identified as the most toxically relevant constituents: however, they did not completely account for the toxic effects of genuine APO and RFO. Interestingly, we found that p-cymene, although per se devoid of toxicity within the tested range of concentrations, was capable of significantly sensitizing U937 cells to the cytotoxic activity of alpha-pinene and alpha-phellandrene, and that specific mixtures of these three terpenes were as toxic as genuine APO and RFO.