Lines of White Leghorn chickens selected > 20 generations for (HA) and against (LA) antibody response to SRBC injected i.v. from 41 to 51 days of age, are now known to have diverged in primary antibody response to SRBC. Experiments described in this dissertation were designed to further evaluate the immune competence of these lines as influenced by age, diet, and a disease agent. A crossing experiment was also conducted to further describe the mode of inheritance of such competence. Humoral immunocompetence was evaluated by primary, memory, and maternal antibody responses to SRBC. Primary antibody response, measured 5, 10, and 20 days after inoculation with SRBC was greater in HA than LA chicks inoculated at 14, 21, and 28 days of age. In chicks injected at 7 days of age, a higher frequency of responders was observed for HA than LA chicks suggesting an earlier onset of immunocompetence in the high than low antibody line. Immunological memory antibody responses (secondary and tertiary) was studied in parallel experiments on two groups of chicks hatched at a 14-day interval. Chicks in both hatches were from the same matings of parental Lines HA and LA. Memory responses were evident in chicks at 14 days of age. Antibody responses to a second and third inoculation with SRBC were similar for both lines suggesting that genetic factors that influence primary and memory responses are not the same. The responses of LA chicks to repeat inoculations with SRBC were anamnestic whereas those of HA chicks initially inoculated at 28 days of age were not anamnestic. This study did not establish any major influence of nutrient density on either primary or memory immune responses even though the higher nutrient density diet improved growth performance. Assays in chicks indicated that maternal antibodies were transferred earlier into eggs laid by HA hens than in those of LA hens ( 7 to 9 days vs 10 to 12 days after inoculation) regardless of dosage administered. Response patterns whether assessed in terms of frequency of detection or magnitude of response showed divergence between the lines. Chicks of parental, reciprocal F , F , and backcrosses of 1 2 mating combinations of Lines HA and LA were injected with SRBC at 36 days of age. Contrasts between parental lines for antibody titers measured 5 and 12 days later showed higher antibody titers in HA than LA chicks. Sex-linked effects were evident because reciprocal contrasts for F crosses, individual heterosis, and 1 maternal heterosis were sex dependent. Response to marble spleen disease virus ( MSDV) measured 6 days after inoculation of chicks from parental, reciprocal F1, F2, and backcross matings of the lines, indicated that the mode of inheritance of spleen weight differed after infection. In the infected chicks, parental contrasts for absolute and relative spleen weights showed greater resistance to MSDV in LA than HA chicks. No other genetic effect was consistently important after infection. / Ph. D.