Abstract
The PR8 strain of influenza virus can remain viable in ageing aerosols for considerable periods (Harper, 1961). However, viability of micro-organisms determined by their growthin vitromay not indicate their ability to infect animals via the respiratory route (Schlamm, 1960; Hood, 1961). If influenza is normally transmitted via this route it is epidemiologically important to establish whether any correlation exists between viability as measured byin vitroorin ovomethods and respiratory infectivity for a susceptible animal host. Previous reports on respiratory infectivity of aged influenza virus aerosols by Edward, Elford & Laidlaw (1943), Loosli, Lemon, Robertson & Appel (1943a) and Loosli, Robertson & Puck (1943b) lack such a comparison.