Contact sensitivity to ‘oxazolone’ (4-ethoxymethylene-2-phenyl oxazolone) can be passively transferred by bone marrow cells. This transfer is abolished by lymphadenectomy 4 days before immunization and reduced by removal of the draining lymph nodes 2 days after immunization. Splenectomy alone has little effect. Bone marrow cells from donors treated with cyclophosphamide 2 days before killing fail to transfer contact sensitivity. It is suggested that the passive transfer by bone marrow cells is due to lymphocytes which have migrated from the draining lymph nodes to the bone marrow.