Immune capacity traits show considerable genetic variation in outbredpopulations. To identify quantitative trait loci (QTLs) for immune capacity inthe pig, various measures of immune function (total and differential leukocytecounts, neutrophil phagocytosis, mitogen-induced proliferation, IL-2 production,and virus induced IFN-alpha production in whole blood cultures, and Ab responsesto two Escherichia coli antigens) were determined in 200 F2 animals from a wildpig-Swedish Yorkshire intercross. The pedigree has been typed for 236 geneticmarkers covering all autosomes, the X chromosome and the X/Y pseudoautosomalregion. Through interval mapping using a least-squares method, four QTLs withsignificant effects were identified; one for total leukocyte counts, one formitogen-induced proliferation, one for prevaccination levels of Abs to E. coliAg K88, and one for Ab response to the O149 Ag. In addition, several putativeQTLs were indicated. The results from the present study conclusively show thatit is possible to identify QTLs for immune capacity traits in outbred pigpopulations by genome analysis.