A strain of human adenovirus type 7, adapted to replication in green-monkey kidney cells, requires the interaction of two particles to initiate plaque formation in the simian cells. One particle is a true adenovirion. The second, apparently defective, consists of a genome carrying amonkey-adapting component in an adenovirus capsid; this genome does not express known SV40 determinants. The addition of human adenovirus type 7 that is not adapted enhances the titer and changesconditions for plaque formation by the adapted virus to a one-particle requirement. Addition of nonadapted human adenovirus type 2 as helper virus results in the transfer of the monkey-adaptingcomponent from adenovirus type 7 to adenovirus type 2. The population containing the adenovirus 2 transcapsidant then has the ability to replicate in simian cells.