Experiments were conducted to study the ontogeny, kinetics, and the influence of aflatoxin B1 on antibody response to sheep erythrocyte (SRBC) antigen in White Leghorn chickens. In the first experiment, chickens from the parental lines and reciprocal crosses between them were fed diets containing graded levels, from 0 to 5697 ppb of aflatoxin B₁. Aflatoxin depressed body weights, feed consumption and feed conversion, with feed conversion being depressed less than either body weight or feed consumption. Although there were no differences among aflatoxin levels for body core temperatures, levels of 1830 ppb and higher caused progressive decreases in surface temperatures. Heterophilia, lymphopenia and reduced liver metabolism were observed at the 5697 ppb level. Although bursa and thymus weights were smaller in the aflatoxin-fed birds, there was no reduction in their SRBC antibody levels. The second experiment was designed to measure primary and secondary antibody response to intravenous immunization of SRBC antigen. Treatments included immunization at the dosage of SRBC antigen under which selection was practiced, and higher and lower concentrations. Although the dosage of primary immunization influenced the magnitude of the secondary response within population-primary dosage correlations between peak primary and secondary antibody response were not different from zero. Differences among populations in antibody levels appeared as early as day 4 and persisted until day 24 post-primary immunization. Yet, the general response patterns were the same for all populations with respective peaks occurring at the same time. The ontogeny of post-hatching production of antibody SRBC antigen and growth of bursa, thymus and spleen were measured in the third experiment. Both parental lines and reciprocal crosses between them reached serological maturity by 14 days of age. By 7 days, there were differences among populations for frequency of responders to SRBC antigen and magnitude of titers, inferring genetic variation for both the event and subsequent levels of antibody production. / Ph. D.