We used polyclonal rabbit anti-Thy-1 antiserum to eliminate T lymphocytes in the peripheral blood, lymph nodes, and spleens but not in the thymus of normal mice. T lymphocytes began to reappear in the peripheral lymphoid compartments 2 weeks after termination of the treatment. By 4 weeks, the numbers of peripheral T lymphocytes had recovered to normal levels. We found that injection of recombinant soluble forms of human CD44 or L-selectin but not human CD8 molecules delayed the reappearance of peripheral T lymphocytes in anti-Thy-1-treated mice. This delay in recovery most likely reflects the ability of these soluble receptors to interfere with lymphocyte migration in vivo. Our results provide in vivo evidence that CD44 and L-selectin regulates lymphocyte trafficking and suggest that soluble forms of both molecules provide useful tools for the study of lymphocyte migration in vivo