Six Ea-B-congenic lines of chickens were used to study the role of the B histocompatibility complex in susceptibility and immunity to Eimeria tenella and E. acervulina. The B haplotypes were 2, 2, 5, 12, 13, or 19. Parental line 15I5 with the B15 haplotype (15I5-15) and a commercial White Leghorn (CWL) line were also used. The B-congenic lines were generally less susceptible to E. tenella than the CWL line based on weight gain, cecal lesion score, plasma pigment, and packed cell volume. Within the congenic lines, the 15I5-15 line was more resistant to E. tenella than the 15.6-2 and 15.7-2 lines. Using a level of infection of E. acervulina resulting in moderate disease, the 15.7-2 line was more susceptible than all other lines based on weight gain, and the 15.6-2 and 15.7-2 lines had greater intestinal lesion scores than the 15.15I-5 or 15I5-15 lines. The CWL line was most susceptible based on lesion score. Thus 15I5-15 chickens were more resistant to primary infection by several criteria than the 15.7-2 or 15.6-2 chickens infected with either coccidial species. A single immunization with 100 oocysts of E. tenella produced less immunity in the congenic lines than in the CWL, whereas four immunizations resulted in immunity in all lines except 15.15I-5. Immunization with E. acervulina produced good immunity to a challenge infection in all lines. These results suggest that the B complex has a detectable influence on resistance and susceptibility to avian coccidiosis, but it may play only a minor role in the development of immunity to a challenge infection.