H-1 virus underwent an abortive cycle of replication in secondary human embryonic lung-cell cultures, characterized by formation of immunofluorescent H-1 antigen, without the production of infective virus. After mixed infection with adenovirus 12, approximately 30 to 40% of cells yielded infective H-1. A single infective adenovirus particle was capable of acting as helper. Comparison of the growth cycles of H-1 and ‘helper’ adenovirus showed that the two viruses had a similar latent period of about 24 hr. Inoculation of cells 24 hr before with adenovirus shortened the latent period for H-1 by at least 12 hr. Particles with serological characteristics of both viruses (mixed coats) were not found in virus yields from mixed infections.

H-1 inhibited yields of adenovirus p.f.u. as well as virus antigens, provided that a low adenovirus input was used. The decrease in adenovirus yield was due to a reduction in the number of cells producing virus. With a high adenovirus input (10 or more p.f.u. per cell), no interference by H-1 was found. Maximal or nearly maximal potentiation of H-1 growth apparently occurred even when the H-1 caused virtually complete interference with formation of its ‘helper’ adenovirus.


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