Alloantibody formation (as measured by the haemagglutination assay) and graft rejection time were measured in HO rats receiving skin grafts from the Ag-B incompatible DA or (HO X DA)F1 strain. Pretreatment of the graft donor with irradiation (1 000 R) or thoracic duct drainage for 5 days, though reducing the level of circulating lymphocytes in these rats drastically, did not impair the host immune responses. Irradiation of graft donors with 1 500 rads did reduce the alloantibody response in the DA leads to HO combination, but this dose also impaired proper graft healing. Lymphocytes injected subcutaneously in small doses (0.3 and 3 X 10(6)) cells) immunized the recipients efficiently for a haemagglutinin response; however, the response developed in a much quicker tempo than the response after skin grafting. Skin grafts varying in size by a factor of approx. 4 evoked alloantibody formation of identical magnitude. On the basis of this and other recent evidence, it is suggested that the antigenicity of rat skin is not critically dependent on the presence of passenger lymphocytes in it.