The aim of this study is to detect quantitative trait loci (QTL) involved in theregulation of the primary and the secondary immune response to sheep red bloodcells (SRBC) in a resource population using microsatellite DNA markers. The F2resource population originates from a cross of two divergently selected linesfor either high (H line) or low (L line) primary antibody response to SRBC. TheF2 population consisted of six half-sib families, three families per each ofreciprocal crosses. Total antibody titres to SRBC were determined byagglutination in serum from all birds. F2, F1 and F0 generations were genotypedfor 170 microsatellite markers, using a whole-genome scan approach. The half-siband the line-cross analyses were performed to determine QTL regions associatedwith regulation of the immune response. In the half-sib analysis, four QTL forSRBC primary response have been identified: on GGA3, GGA5, GGA16 and GGA23. NoQTL was identified for SRBC secondary response under the half-sib model. In theline-cross analysis, three QTL were identified on GGA10, GGA16 and GGA27 forSRBC primary response and five QTL were identified on GGA6, GGA9, GGA15, GGA16and GGA27 for SRBC secondary response. Subsequently, the family contribution ofindividual families to the QTL was analysed. The family with the largestcontribution was genotyped with additional microsatellite markers in the QTLregion on GGA5. The extended half-sib analysis with additional genotypeinformation results in narrowing down the QTL region on GGA5.