Monkey kidney cultures in monolayer were doubly infected with high multiplicities of CA9 and E7 viruses. During the latent period, the infected cells were suspended, diluted, and distributed under oil into droplets each containing a single cell, as checked by microscopic observation.

The virus particles released by individual cells into the microdrop were characterized in differential plaque neutralization tests. Fifteen per cent of the microdrops contained doubly neutralizable particles, 53 per cent yielded either CA9 or E7 particles, and 34 yielded particles of an intermediate character (deficits between 37 and 75 per cent).

On passage, the doubly neutralizable particles yielded progeny of both parental types. All passage strains behaved like the corresponding parent strain as regards pathogenicity for newborn mice, which is to say that this property was limited to virus particles with CA9 antigenicity.

Coxsackie A9 has a more rapid growth cycle than ECHO 7 in rhesus monkey kidney cell cultures, and a slower one in patas cultures. In rhesus, when E7 virus was added first, CA9 could be added up to 2 hours later, and still a significant number of cells yielded either CA9 or doubly neutralizable virus. The converse was observed in patas cells.