Butel, Janet S. (Baylor University College of Medicine, Houston, Tex.), and Fred Rapp. Replication in simian cells of defective viruses in an SV40-adenovirus “hybrid” population. J. Bacteriol. 91:278–284. 1966.—An SV40-adenovirus type 7 “hybrid” virus population, previously shown to contain two viruses capable of complementation in green monkey kidney (GMK) cells, has a growth cycle in GMK cells similar to that of adenovirus type 7 in the presence of SV40. Extending previous preliminary results, the addition of adenovirus types 2, 7, or 12 to monolayers of GMK cells enhanced plaque formation by the SV40-adenovirus hybrid by as much as 200-fold. The terminal enhanced plaques, initiated by the hybrid in the presence of helper adenovirus, were found to contain progeny which could induce the synthesis of SV40 tumor antigen but which were coated with the protein of the helper adenovirus, type 2, 7, or 12, respectively. The particle carrying the SV40 tumor antigen determinant, named PARA, is defective in that it cannot direct the synthesis of capsid protein; information for the coat for PARA is supplied by the adenovirus. One-step growth curves of the hybrid virus population in monkey cells revealed that synthesis of both types of particles, adenovirus and PARA, proceeds at a similar rate, with a latent period of 16 to 20 hr being followed by an exponential increase in titer during the following 20 hr. Maximal titers for both particles were obtained 48 hr after inoculation of the cultures. Neither the PARA nor the adenovirus component replicated in GMK cells in the absence of the other.