The effects of two host genes on retrovirus-induced murine lymphoma were evaluated by studying 114 F2 intercross mice between SL/Kh and AKR/Ms mice. Out of 47 T-lymphoma-bearing F2 mice, 45 had the AKR-derived dominant allele at Tism-1. The length of the lymphoma latent period was not related to type of tumor. Instead, it was significantly shortened by a recessive SL/Kh-derived allele at a major histocompatibility complex (MHC)-linked locus on Chr. 17. A quantitative trait analysis of the latent period yielded a maximal logarithm of likelihood ratio for linkage (LOD) score of 7.06 at a class II gene within MHC. The SL/Kh-derived recessive gene was named lla (lymphoma latency acceleration).