Serum insulin-like growth factor-1 (IGF-1) and femoral bone mineral density (BMD) differ between two inbred strains of mice, C3H/HeJ (C3H) and C57BL/6J (B6), by approximately 30% and 50%, respectively. Similarly, skeletal IGF-1 content, bone formation, mineral apposition, and marrow stromal cell numbers are higher in C3H than in B6 mice. Because IGF-1 and several bone parameters cosegregate, we hypothesize that the serum IGF-1 phenotype has a strong heritable component and that genetic determinants for serum IGF-1 are involved in the regulation of bone mass. We intercrossed (B6 x C3H)F1 hybrids and analyzed 682 F2 female offspring at 4 months of age for serum IGF-1 by radioimmunoassay and femoral BMD by peripheral quantitative computerized tomography (pQCT). Genomic DNA was assayed by polymerase chain reaction (PCR) to determine alleles for 114 Mit markers inherited in F2 mice at average distances of 14 centimorgans (cM) along each chromosome (Chr). Serum IGF-1 levels in the F2 progeny were relatively normal in distribution, but showed a greater range than either progenitor, indicating that serum IGF-1 level is a polygenic trait with an estimated heritability of 52%. Serum IGF-1 correlated with femoral length (r = 0.266, p < 0.0001) and femoral BMD (r = 0.267, p < 0.0001). Whole genome scans for main effects associated with serum IGF-1 levels revealed three significant QTLs (in order of significance) on mouse Chrs 6, 15, and 10. The QTL on Chr 6 showed a significant reduction in IGF-1 associated with increasing C3H allele number, whereas the Chr 15 and Chr 10 loci showed additive effects with increasing C3H allele number. A genome-wide search for interacting marker pairs identified a significant interaction between the Chr 6 QTL and a locus on Chr 11. This interactive effect suggested that when the Chr 11 locus was homozygous for C3H, there was no effect of the Chr 6 locus on serum IGF-1; however, the combination of C3H alleles on Chr 6 with B6 alleles on Chr 11 was associated with reduced serum IGF-1 concentrations. To test this in vivo, we tested congenic mice carrying the Chr 6 QTL region from C3H on a B6 background (B6.C3H-6). Both serum IGF-1 and femoral BMD were significantly lower in female congenic than progenitor B6 mice. In summary, we identified three major QTLs on mouse Chrs 6, 10, and 15, and noted a major locus-locus interaction between Chrs 6 and 11. We named these QTLs IGF-1 serum levels (Igf1sl1 to Igf1sl4). Functional isolation of the Igf1sl1 QTL on Chr 6 for IGF-1 in B6.C3H-6 congenic mice demonstrated effects on both the IGF-1 and BMD phenotypes. The genetic determinants of these Igf1sl QTLs will provide much insight into the regulation of IGF-1 and the subsequent acquisition of peak bone mass.