The influence of the maj or histocompatibility (B) complex on acquired immunity to the avian coccidium Eimeria tenella was studied in 217 F 4 segregants (BZB 2, BZB 5, BSB 5) of a cross between inbred lines 61 (BZB z) and 151 (BSB 5) and segregating haplotype combinations of UNH105 (BZ3B 23, BZ3B 2~, BZ4BZ4), a noninbred line of New Hampshire chickens. Chickens were immunized at 6 weeks of age with 500 oocysts daily for 5 days, then challenged 14 days later with 10 000 oocysts. Responses to infection were evaluated by cecal lesion scores, body weight gain, delayed wattle reaction (DWR), and spleen weight. The F4 segregants of genotypes BaB 5 and BSB s exhibited greater immunity to challenge than B2B 2 chickens. BSB 5 chickens showed a significantly greater DWR following immunization and larger spleens 6 days after the challenge than either of the other genotypes. However, both BZB 5 and BSB 5 chickens demonstrated significantly lower lesion scores than B2B 2 chickens. There were no significant differences in weight gain among these genotypes. Among 139 line UNH105 segregants, B23B 23 hosts had significantly lower lesion scores than B24B 24 chickens. No other differences in immune response among line UNH 105 genotypes were detected.