The objective of this project is to determine the factors which influence lymphocyte traffic to the lymphoid organs following antigenic challenge. It is known that a variety of antigens, tumors, allografts, adjuvants, and GVH reaction can induce alterations in lymphocyte homing patterns at the site of antigenic stimulation. Such exposure to antigen in vivo usually results in increased localization of cells, termed "trapping", in spleens, or lymph nodes, depending upon the route of antigen administration. Since it is known that lymphocyte trapping is a T-cell dependent phenomenon, the question of whether trapping is subject to T-cell regulatory mechanisms was asked. Recipient animals of different ages were subjected to immunosuppressive treatments such as administration of ATS, cortisone, or x-ray, prior to challenge with antigen. The magnitude of Cr51 labelled lymphocyte migration to recipient lymphoid organs was then quantitated in immunized, tolerized, or control animals.