I have measured the proportion of host cells in a popliteal lymph node undergoing a graft-versus-host (GVH) reaction. By using cytotoxic alloantisera with specificity for donor and host Ag-B determined antigens, it was found that about 90% of live cells suspended from the node were of host origin. The role of host T lymphocytes in this reaction was evaluated by investigating GVH-reactivity in hosts depleted of T cells (B rats). These rats differed very little from normal hosts with regard to lymph node weight increase (which was, in fact, slightly enhanced), increase in node cellularity, and donor/host contribution. The number of i.v. infused 51Cr-labelled thoracic duct lymphocytes localizing in popliteal lymph nodes undergoing a GVH-reaction was increased, but the significance of this phenomenon for the lymph node enlargement is questionable.