Sixteen B10.W congenic lines carrying on C57BL/10Sn or B10 background H-2 haplotypes extracted from wild mice are described. The lines were tested serologically in two ways: first by antisera against known H-2K and H-2D private antigens of inbred strains, and second by antisera made in inbred strains against antigens carried by the B10.W lines. Both direct cytotoxicity and absorption tests were used. The analysis resulted in the serologic identification of the H-2 haplotypes carried by the 16 B10.W lines. Lines B10.STA10 and B10.STA12 are serologically indistinguishable; so are lines B10.KPA42, B10.KPA132, and B10.SNA57, whose H-2 haplotypes resemble the H-2v haplotype of B10.SM; all other B10.W lines are different from one another and different from all inbred strains known. In total, 10 new H-2 haplotypes (H-2w11 through H-2w20) and 16 new H-2 antigens (H-2.110 through H-2.125) were identified. One haplotype was discovered carrying new combination of known H-2 antigens: H-2w19, which is H-2K.19, H-2D.2. Known H-2 antigens in combination with new ones were discovered in haplotypes H-2w8 (H-2.23 and 110), H-2w12 (H-2.23 and 112), and H-2w11 (H-2.26 and H-2). The first two of these three haplotypes are probably the result of intra-H-2 crossing over that occurred during the production of B10.W lines; the last one (and the H-2w19 haplotype) is probably a natural recombinant.