Antialloantibodies were prepared in F1 hybrid rats by immunization with alloantibodies from one parent raised against antigens of the other parent. The Ig fraction of such antialloantibodies was iodinated and used to investigate the nature of idiotype-carrying normal lymphoid cells. Lymphoid cell populations of the proper genotype fixed radioactive antialloantibody in proportion to their B-cell content. Activated T-cell populations, when depleted of B cells, did not fix significant amounts of radioactivity. Idiotype-carrying cells were sensitive to rabbit antirat Ig serum lysis and to antialloantiserum lysis, but not to rabbit anti-T-serum lysis. Also, the normal idiotype-containing B cell could be shown to have the expected antigen-binding specificity of its receptor. This was shown by the use of fibroblast cell monolayers that adsorbed specifically those cells capable of fixing the proper antialloantibody.