The transfer of homozygous C57Bl/6 (B6) or DBA/2 (DBA) parental strain T cells into normal B6D2F1 mice in the parent-into-F1 (p → F1) model results in a graft-vs.-host disease (GVHD) that takes one of the following two forms: (a) acute GVHD seen with B6 → F1 mice and mediated by donor CD8 cytotoxic T cells that eliminate host lymphocytes and (b) a chronic lupus-like GVHD seen with DBA → F1 mice and mediated by donor CD4 T cell cognate help to autoreactive B cells resulting in autoantibody production and renal disease similar to human lupus. Importantly, these two phenotypes can be distinguished by flow cytometry as early as 2 weeks after donor cell transfer. The p → F1 model can be used to screen for agents that alter lupus development. Additionally, the model is useful for preclinical screening of biologic agents with immunomodulatory potential. Agents that selectively inhibit CD8 T cell function will convert acute GVHD to chronic GVHD in B6 → F1 mice. Conversely, agents that promote CD8 CTL function will convert chronic GVHD to acute GVHD in DBA → F1 mice. Agents that completely suppress T cell function will block both phenotypes. The model is also useful for examining the effects of T cell mutations by transferring mutant T cells into wild-type hosts and assessing the effects on disease phenotype. Differences observed from wild-type T cells → F1 can be directly ascribed to alterations in mutant T cell function. Because of the early 2-week phenotype development, the p → F1 model is well suited to screening of potential immunomodulatory therapeutic compounds and the assessment of T cell mutations on in vivo function.