Six experiments were carried out with aerosol immunization, using various doses of the La Sota vaccine against Newcastle disease, and a control infection with massive amounts of the viscerotropic virus strain Pavlikeni to evaluate the duration of carrying the Newcastle disease virus. The aerosol vaccination of the birds was performed in laboratory conditions in an aerosol chamber of a flow-dynamic type of 450 l. The amount of the virus in the aerosol was determined through virologic investigations of samples taken by means of a glass capillary impinger. It was found that a short-term carrier state set in up to the 9th-10th day with the La Sota strain, regardless of the dose used and the age of the birds. The persistence of the velogenic virus in the organs of birds depended on the amount of vaccine used and the age of the birds. With 2-month-old chickens at optimal dosing of the vaccine resistance could be built up to the infection. When low doses of the vaccine were used with 10-day-old chicks as well as when over-dosing occurred the strain Pavlikeni could be recovered from the organs in the course of 3 to 7 days.