The influence of microenvironment on the course of CD8⁺ T cell responses in vivo was investigated by injecting H-2K^b-specific T cells from donor TCR transgenlc (TCR-Tg) mice into H-2K^b-tag mice. H-2K^b expression in recipients was either ubiquitous (CBK mice) or restricted to myeloid and erythroid cells (Kβ mice). Donor T cells proliferated as extensively and acquired similar surface phenotypes in spleen of both recipient types. Thus, neither the restricted pattern of H-2K^b expression nor the significantly reduced level of H-2K^b expression by myelold cells in Kβ recipients affects the ability of the splenic microenvironment to prime T cell proliferation in vivo. However, an unsustained burst of cytolytic activity was generated rapidly in spleen of CBK recipients, whereas relatively little cytolytic activity was generated in Kβ spleen. This indicates that effector T cells were not generated efficiently in spleen of Kβ recipients even though extensive T cell proliferation was taking place in this microenvironment. Furthermore, activated donor T cells dispersed rapidly throughout primary and secondary lymphoid organs of Kβ recipients, whereas few T cells migrated from spleen in CBK recipients. Consequently, the course of CD8⁺ T cell responses and the anatomical distribution of activated T cells are profoundly influenced by the nature of the antigenlc microenvironment encountered in vivo. We conclude that T cells rapidly proliferate and acquire new tissue-homing characteristics but do not differentiate into cytolytic effector cells at the site of priming when they encounter myelold cells expressing low levels of antigen in vivo.