(1) The technique of chorioallantoic membrane inoculation has been applied to the study of the virus of infectious laryngotracheitis as it occurs amongst Australian poultry. (2) When suitably diluted suspensions of virus are inoculated, isolated foci or pocks appear whose macroscopic form and histological structure is characteristic. The numbers of these foci may be used as a measure of the amount of virus present. (3) Two distinct types of focus are produced by laryngotracheitis strains, one being characteristic of epizootic strains from New South Wales and from America, the other of a Victorian strain which is of very low virulence for fowls. (4) No qualitative antigenic differences can be detected amongst these strains but the epizootic strains are more readily neutralised by immune serum than the enzootic Victorian strain. (5) A study of the inactivation of the virus by immune serum shows that (a) the process of inactivation requires time for its completion in vitro; (b) the proportionate reduction in titre produced by a given concentration of antiserum is independent of the initial virus concentration; (c) reactivation by dilution is readily demonstrable; (d) virus in the presence of small concentrations of immune serum producing only a slight inactivating effect is rendered incapable of passing a gradocol membrane normally permeable to it; (e) the foci produced from partially neutralised virus suspensions are smaller than normal, suggesting delay in the initiation of foci. (6) These findings bring the neutralisation of a typical virus by immune serum completely into line with the phage-antiphage reaction as described by Andrewes and Elford.