Quantitative trait loci for BW at 4, 6, 8, 12, and 18 wk of age were detected inan experimental F2 cross of layers divergently selected for primary antibodyresponse to SRBC. A negative phenotypic correlation between levels of antibodytiters and BW, was reported earlier within founder lines. The entireexperimental population was genotyped with 174 microsatellite markers equallydistributed over the genome. Two genetic models were applied in the QTLanalysis: a half-sib model and a line-cross model, both using the regressioninterval method. In the half-sib model, 3 QTL for BW at 4 wk of age onchromosomes GGA2, GGA3, and GGA9 were detected. For BW at 6 wk of age, 3 QTLwere detected on GGA2, GGA3, and GGA6. For BW at 8 wk of age, a QTL was detectedon GGA7, and for BW at 12 and 18 wk of age, a QTL was detected on GGAZ. With theline-cross analysis model, one QTL on GGA7 for BW at 4 wk of age was detected.Two QTL were detected on GGA3 and GGA7 for BW at 6 wk of age, and one on GGA3was detected for BW for 8 and 12 wk of age. For BW at 18 wk of age, there wereno QTL under the line-cross analysis model. The present data suggest that 1) adifferent set of genes affects the early and the late growth, and 2) genesselected to humoral immune responsiveness are different from genes underlyinggrowth.