In field trials approximately 300,000 birds were vaccinated against Marek's disease (MD) by aerosol spray with 1,000 to 5,000 PFU of cell-free HVT vaccine per chick. Chicks were sprayed in the trays of the hatchers which still contained dust and down after hatching. Prior to spraying, the incoming and outgoing air ducts were closed. The chicks were sprayed for 12--15 min. with HVT in 250 to 300 ml. of diluent, and after completion of spraying the chicks remained in contact with the aerosol for approximately 10 minutes. In birds vaccinated by aerosol spray the incidence of MD ranged from 0.0% to 24.4%; while the incidence of MD in birds vaccinated by subcutaneous inoculation varied from 0.02 to 23.9%. In trial 1 the incidence of MD in birds vaccinated by aerosol spray was essentially the same as those vaccinated by subcutaneous inoculation; whereas, the incidence of MD (Trial 2 and 3) was higher in birds vaccinated by aerosol as compared to chickens vaccinated by subcutaneous inoculation.