Complex traits are under the genetic control of multiple genes, often with weak effects and strong epistatic interactions. We developed two new collections of mouse strains to improve genetic dissection of complex traits. They are derived from several backcrosses of the Mus spretus SEG/Pas or STF/Pas strains on the C57BL/6J background. Each of the 55 interspecific recombinant congenic strains (IRCSs) carries up to eight SEG/Pas chromosomal segments with an average size of 11.7 Mb, totalizing 1.37% of the genome. The complete series covers 39.7% of the SEG/Pas genome. As a complementary resource, six partial or complete interspecific consomic strains were developed and increased genome coverage to 45.6%. To evaluate the usefulness of these strains for QTL mapping, 16 IRCSs were compared with C57BL/6J for seven hematological parameters. Strain 66H, which carries three SEG/Pas chromosomal segments, had lower red blood cell volume and higher platelet count than C57BL/6J. Each chromosomal segment was isolated in a congenic strain to evaluate individual effects. Congenic strains were combined to assess epistasis. Our data show that both traits were controlled by several genes with complex epistatic interactions. IRCSs are therefore useful to unravel QTL with small effects and gene-by-gene interactions.