Abstract AS YET there is no critical evidence to show definitely that the agent of avian leucosis is or is not transmitted through the egg. There has, however, been accumulated considerable circumstantial evidence to support both possibilities, especially the former concept. This is not unexpected, because positive data are more likely to be published than are negative data. It is obvious that for a complete understanding of the avian leucosis complex more information as to the possibility of egg-borne transmission of the etiological agent must be provided. In order for the disease to develop, two conditions must be satisfied. The infective agent must be present and the host must be genetically susceptible. Natural (genetic) resistance is a relative factor and its normal expression can be overridden by an excess of the agent or exposure. Such happenings occur regularly and are responsible for variations in incidence of the disease within a flock . . .