We herein report new evidence that the QTL effect on chromosome 20 in FinnishAyrshire can be explained by variation in two distinct genes, growth hormonereceptor (GHR) and prolactin receptor (PRLR). In a previous study inHolstein-Friesian dairy cattle an F279Y polymorphism in the transmembrane domainof GHR was found to be associated with an effect on milk yield and composition.The result of our multimarker regression analysis suggests that in FinnishAyrshire two QTL segregate on the chromosomal region including GHR and PRLR. Bysequencing the coding sequences of GHR and PRLR and the sequence of three GHRpromoters from the pooled samples of individuals of known QTL genotype, weidentified two substitutions that were associated with milk production traits:the previously reported F to Y substitution in the transmembrane domain of GHRand an S to N substitution in the signal peptide of PRLR. The results providestrong evidence that the effect of PRLR S18N polymorphism is distinct from theGHR F279Y effect. In particular, the GHR F279Y has the highest influence onprotein percentage and fat percentage while PRLR S18N markedly influencesprotein and fat yield. Furthermore, an interaction between the two loci issuggested.