Bone marrow cells from viable motheaten (me(v)) mutant mice fail to repopulate the thymus of irradiated recipients upon i.v. adoptive transfer. This defect can be corrected by the addition of wild-type marrow to the me(v)/me(v) marrow prior to i.v. injection. The present study focused on the nature of this apparent defect in me(v)/me(v) prothymocyte activity. We established an adoptive transfer assay system that facilitated the investigation of the migration patterns of wild-type and me(v)/me(v) marrow prothymocytes in adoptive recipients. Wild-type prothymocytes were found to seed the thymus of recipients as early as 1 day after i.v. injection. This early wave of prothymocyte seeding peaked at 4 days and diminished to undetectable levels at 6 days postinjection. After an interval of no detectable wild-type precursor activity in the thymus of primary recipients, a second wave of activity was observed on Day 15. This second wave resulted in permanent engraftment and persisted throughout the observation period. In contrast, only one initial wave of me(v)/me(v) prothymocyte seeding occurred in the recipient thymus. Detection of mev/mev precursors in the recipient's thymus was dependent upon the coinjection of wild-type marrow cells. These results suggest that me(v)/me(v) prothymocytes initially seed to the thymus of adoptive recipients, but fail to proliferate and/or differentiate due to a defect in marrow-derived intrathymic accessory cell activity.