Flavonoids are abundant in plants as secondary metabolites where they have antioxidant activities. The diverse and non-specific inhibition of several enzymes, including avian myeloblastosis virus reverse transcriptase (RT), human immunodeficiency virus (HIV) RT, murine leukemia virus RT, HIV-1 protease, trypsin and elastase, were tested for by observing the effects of several flavonoid compounds. Some of the flavonoids were reconfirmed as antiretroviral agents as previously shown by others. Flavonoids belonging to the flavonol or flavone group simultaneously inhibited reverse transcriptases and proteases. On the contrary, flavonoids belonging to the flavanol, isoflavone or flavanone group were less likely to inhibit reverse transcriptase or protease. Vegetables and fruits rich in flavonoids have become integral and even necessary in the animal diet, suggesting that flavonoids may have developed to play beneficiary roles for not only the plants themselves but also the predator animals.