A comparison has been made between the alloantibody response evoked by graded doses of cells being allogeneic and semiallogeneic with the host. The responses were measured by hemagglutination and complement-dependent lymphocytotoxicity. In the strain combination of rats used (DA ± HO), a marked difference in the optimal dose was found after i.v. injection of lymphocytes. Although semiallogeneic cells gave the better antigenic stimulus at lower cell doses (0.3 and 3 x 106 cells), allogeneic cells were better at higher cell doses (30 and 150 x 106 cells). Immunization experiments with allogeneic and semiallogeneic erythrocytes or mitomycin- or heat-treated lymphocytes indicated that the antigenicity of the cells was not directly related to the cell surface concentration of antigen and was independent on the proliferative capacity of the lymphocytes; it was, however, curtailed by heat treatment of these cells. The 3 x 106, but not the 0.3 or 30 x 106 dose of semiallogeneic cells primed efficiently for a secondary hemagglutinin and cytotoxic response. I.v. injected 51Cr-labelled allogeneic and semiallogeneic lymphocytes showed different localization patterns. Between 4 and 24 hr after injection allogeneic lymphocytes were apparently more rapidly lost from the recipient lymph nodes. Collectively, these data indicate that the different immunizing properties of i.v. injected allogeneic and semiallogeneic lymphocytes are not simple consequences of the antigen dose transferred, but suggest that different localization patterns might influence the ability of these cells to induce cytotoxic and hemagglutinating alloantibody formation.