The isolation and identification of deoxypodophyllotoxin and a new lignan, 5'-desmethoxydeoxypodophyllotoxin, from the dried exudate of Bursera morelensis (Burseraceae) are reported. Deoxypodophyllotoxin showed high activity in the KB and PS test systems; the new lignan, although highly active against the KB test system, demonstrated only marginal activity against the PS test system. A structure is suggested for the new lignan, which was named morelensin.