Five different lines of a strain 2 guinea pig leukemia (L2C) which had been carried in different laboratories share certain chromosomal markers and have a common surface immunoglobulin idiotypic determinant indicating that they have a common origin. All these leukemic lines have on their surface of the B alloantigen (equivalent of the murine H-2K and H-2D antigens) and four of these five lines have on their surface the Ia alloantigens normally present on the strain 2 lymphocytes. The result of a study of the growth and rejection patterns of these leukemias in inbred and random-bred guinea pigs of selected histocompatibility type indicates that both the B and Ia antigens can act as transplantation antigens in guinea pigs. Immunization protection tests in syngeneic animals demonstrated that the four Ia-positive leukemias possessed a tumor-associated transplantation antigen (TATA), while the one Ia-positive leukemias possessed a tumor-associated transplantation antigen (TATA), while the one Ia-negative leukemia by this criteria did not appear to have TATA. However, crisscross immunization protection tests demonstrated that preimmunization of syngeneic animals with an Ia-positive L2C line lead to a subsequent protection against challenge with the Ia-negative leukemia. Immunization with the Ia-negative line never protected against a subsequent challenge with any of the leukemic cells of L2C lines. These results strongly suggest that the Ia-negative leukemia possessed a TATA that can be recognized but is not itself immunogenic, and also indicate that Ia antigens on L2C cells are functionally associated with TATA and can act as immunological carries for tumor transplantation determinants.