Chickens from lines selected for high (HA) and low (LA) antibody response to sheep erythrocyte antigens were used to study relationships between blood group systems and resistance to Eimeria tenella. Results of natural and controlled exposures to E. tenella showed Line HA chickens to be more resistant than those from Line LA. Differences in resistance to E. tenella were found among alleles for the I blood group system with degree of resistance depending on the background genome.