RESUMEN
1. A total of 3236 females from eight meat-type strains, half of which were vaccinated for Marek's disease (MD), and 11,193 Leghorn females from ten strains, all vaccinated for MD, were adventitiously exposed to MD and lymphoid leukosis (LL) viruses and observed to 392 and 497 d of age, respectively. 2. In the meat-type birds, vaccination reduced total mortality from 43-4% to 27-1% and mortality due to MD from 16-4% to 5-4% but did not affect mortality and LL (2-9% and 3-4%). 3. In the vaccinated Leghorns total mortality was 11%, including 2-1% from MD and 1-2% from LL. 4. Significant differences between strains of chickens were found in total mortality, as well as in MD and LL mortality. 5. Strain by vaccination interaction was observed in total rearing and adult mortality, as well as in the MD mortality of adult meat-type females. 6. Leghorn strains with higher rate of egg production and meat-type strains with lower growth rate to have better viability.