The purpose of this work was to elucidate the genetic fine structure of the central portion of mouse chromosome (Chr) 2. Seven Chr 2 congenic mouse strains [B10.PA(L)-pa we un at, B10.PA(L)-pa Aw, B10.PA(L)-we un at, B10.PA(J)-pa a, B10.FS-we Aw, B10.C-we Aw, and B10.YBR-a] were produced. Breeding studies were carried out using strains B10.PA(L)-pa we un at and B10.LP-H-13b to accurately determine the recombination frequencies between marker genes pa and we (1.9% +/- 0.3), we and un (8.8% +/- 0.5), and un and at (4.5% +/- 0.4) of strain B10.PA(L)-pa we un at. These strains and other Chr 2 congenic strains were typed for immunologically defined loci using monoclonal antibody (mAb) C23 reactive with the gene product of B2mb T-lymphocyte clone C1 reactive with the gene product of H-3a and H-3c, and lymphocyte clone H1.8 reactive with the gene product of Hd-1a. B2m and H-3 typing located a recombinational event separating [pa B2m H-3] from we (the order of bracketed genes is not known). Hd-1 typing indicated that Hd-1 maps distal to [H-42, H-44] and proximal to un. The gene order [pa, B2m, H-3], we, [H-42, H-45], Hd-1, un, H-13, at, with H-44 mapping centromeric to Hd-1, is indicated by the data.