The cytotoxic effects of sesquiterpene gamma-lactones in tissue culture were studied on two different cell lines, one of them fibroblastoid cells L-929 from mice areolar tissue, and the other epithelial cells HEp-2 from human laryngeal carcinoma. Ten lactones were tested, all of them having in their molecule an alpha methylene gamma lactone group, which had been described as conferring cytotoxic activity. Five of these lactones have a furanic ring in their molecule whose presence gives them a higher cytotoxic activity. Different doses of each lactone were tested. Cell number and exclusion tests were simultaneously performed per dose, obtaining for all of them the ED50, which was the calculated as the effective dose inhibiting growth to 50 per cent of control growth. The ED50 was about 1 to 5 microgram in those lactones with furanic ring. The two cell lines had a different response to the same lactone.