Representative human and simian immunodeficiency viruses (HIV/SIVs) have been monitored for their Vif and Vpu activities in a wide variety of cells. In contrast to the prototype HIV-J, viruses of the other groups do not necessarily have these activities. Only HIV-2 and SIVmnd were clearly demonstrated to show the Vif and Vpu activities, respectively. Tho exchangeability of these accessory activities between viruses was then assessed to determine the relatedness of the viruses. Quite different from the results for Tat and Rev trans-activators, the activities are almost fully compatible between viruses. These results may facilitate the functional grouping of various HIV/SIVs.