Previous reports have stated that activated lymphocytes are more motile than unsensitized lymphocytes and that certain subpopulations of lymphocytes are more motile than others. In order to determine at what stage of activation lymphocytes become motile, we have examined lymphocyte motility as well as other functions at various times after mitogenic or allogeneic stimulation in culture. Lymphocytes cultured with concanavalin A (Con A) or with alloantigen become most motile after peak thymidine incorporation has occurred. Allosensitized lymphocytes need not possess cytolytic capacity in order to become motile. Allosensitized lymphocytes retain the ability to locomote at near maximal rates for at least 2 weeks after stimulation. When these motile, allosensitized lymphocytes are recultured in the presence of fresh irradiated stimulator cells, they lose their ability to locomote during active thymidine incorporation and become motile again when thymidine incorporation has abated. Lymphocyte clones demonstrated motility cycles similar to cycles observed with bulk cultures (i.e., peak motility is observed only after peak thymidine incorporation has occurred in the subculture). All T lymphocyte clones tested are motile, regardless of Lyt phenotype or function, if tested for motility on the appropriate day of subculture. Studies of clone motility at intervals less than 24 hr after restimulation revealed that motility ceased before tritiated thymidine uptake occurred. The migratory potential of lymphocytes in vitro does not seem to depend on phenotype or the nature of the mitogenic stimulus. It seems instead to be part of the cyclic response of the cell to activation, with early inhibition of motility and subsequent recovery of locomotor function. Whether ability to locomote is intrinsic to the activated cell or depends on environmental factors requires further investigation.