The Single Comb White Leghorn, the Cornell S strain chickens vaccinated at one day of age with cell free Herpesvirus of turkey (HVT) 100 and 1000 focus forming-units (FFU) per bird were raised in isolatores. At different periods of time the vaccinated and control chickens were reared together with chickens with clinical symptoms of Marek's disease previously inoculated with JM-Strain of Marek's disease virus (MDV) in a separate isolation room throughout the experiment. The specific protective effect was 84.6-100 percent in vaccinated chickens raised together with chickens with MD, soon or 4, 7, 11, 14, 17 and 21 days after vaccination. The period of MD evolution, Mean Time to Death (MTD) at unvaccinated chickens was 54.8 and at vaccinated chickens with 100 and 1000 FFU of HVT per bird, MTD was 65.9 and 78.5 days respectively. Rhode Island Red (RIR-HPRS) and Brown Leghorn (BrL-HPRS) chickens vaccinated at one day of age with cell-free HVT 100 and 1000 FFU per bird were raised in isolation rooms. At 21 day of age vaccinated and unvaccinated chickens were challenged with MDV strain P-501 and reared together in isolation rooms throughout the experiments. The specific protective effect was 71.6-100 percent and 76.8-100 percent at vaccinated RIR-HPRS and BrL-HPRS respectively. Mean time to death was 58-68.5 and 95 days at RIR-HPRS vaccinated chickens with 100 and 1000 FFU of HVT respectively and 38.1-48.25 days at control group. MTD was 72.5-87 days at BrL-HPRS chickens vaccinated with 100 FFU of HVT and 41.5-50.25 days at control group.