A linkage disequilibrium approach with microsatellites was employed toinvestigate QTL affecting immune response. Highly inbred males of twoMHC-congenic Fayoumi chicken lines were mated with highly inbred G-B1 Leghornhens. Adult F2 hens (n = 158) were injected twice with SRBC and fixed Brucellaabortus (BA). Agglutinating antibody titers were measured. Secondary phaseparameters of maximum titers (Ymax) and time (Tmax) needed to achieve Ymax wereestimated from postsecondary titers by using a nonlinear regression model. Athree-step genotype strategy (DNA pooling, selective genotyping, and wholepopulation genotyping) was used to identify microsatellite markers that areassociated with immune response to SRBC and BA. The linkage distances betweenadjacent markers in the F2 population were estimated by Crimap. The QTLaffecting immune response to SRBC and BA were detected based on F statistic byinterval mapping. A total of five significant QTL, as determined by apermutation test, were detected at the 5% chromosome-wise level on Chromosomes3, 5, 6, and Z. Two (Chromosome 3 and 6) of five QTL were significant at the 1%chromosome-wise level. The variance explained by the QTL ranged from 6.46 to7.50%. The results suggest that regions on Chromosomes 3, 5, 6, and Z containQTL that affect antibody kinetics in the hen.