MATERIALS AND METHODS

ABSTRACT Three inbred lines of White Leghorns were crossed in all combinations giving nine mating types which were exposed to Marek's disease (MD) by natural and artificial means. Ranking of the mating types by MD mortality after artificial exposure to either the JM or the GA isolate was similar to the ranking after natural exposure, sug-gesting that exposure to either isolate would be effective in increasing mortality for selec-tion of lines resistant to naturally occurring disease. Differences in the response pattern between inoculation and contact exposure to either isolate was noted. This observation, the large maternal effects and the changes in dominance due to method of exposure, indicate that cross performance cannot be predicted from pureline performance, and that each cross should be evaluated under several conditions of exposure. POULTKY SCIENCE 51: 261-267, 1972 APREVIOUS report (Crittenden et al., 1972) outlined diallel cross ex-periments designed to investigate the ge-netic control of resistance to lymphoid leu-kosis (LL) and acute neoplasms induced by the avian leukosis-sarcoma (L-S) vi-ruses. The present paper analyzes genetic resistance to naturally occurring and in-duced Marek's disease (MD) in chicks produced by the matings described in the report referred to above