Pim-1 is an oncogene activated in mouse T-cell lymphomas induced by Moloney and AKR mink cell focus (MCF) viruses. Pim-1 was previously mapped to chromosome 17 by somatic cell hybrids, and subsequently to the region between the hemoglobin α-chain pseudogene 4 ( Hba-4ps) and the α-crystalline gene ( Crya-1) by Southern blot analysis of DNA obtained from panels of recombinant inbred strains. We have now mapped Pim-1 more accurately in t-haplotypes by analysis of recombinant t-chromosomes. The recombinants were derived from Tt s6tf t 12 parents backcrossed to + tf +tf , and scored for recombination between the loci of T and tf. For simplicity all t-complex lethal genes properly named tck-t x are shortened to t x . The Pim-1 gene was localized 0.6 cM proximal to the t w12 lethal gene, thus placing the Pim-1 gene 5.2 cM distal to the H-2 region in t-haplotypes. Once mapped, the Pim-1 gene was used as a marker for further genetic analysis of t-haplotypes, t w12 is so close to tf that even with a large number of recombinants it was not possible to determine whether it is proximal or distal to tf. Southern blot analysis of DNA from T-tf recombinants with a separation of t w12 and tf indicated that t w12 is proximal to tf. The mapping of two alletic t-lethals, t 0 and t 6 with respect to t w12 and tf has also been a problem. The map position of t 0( t 6) was obtained through Southern blot analysis of DNA from a panel of Tt s6tf t 0 and Tt s6tf t 6 recombinants with separation of t w12 and t 0( t 6). t 0( t 6) was placed distal to tf. Southern blot analysis of the t h20 deletion, which covers the locus of tf, indicated that this deletion does not include the Pim-1 gene.