Long-lasting and stable lymphoid cell chimerism has been noted in three littermates of a group of five rabbits given injections at birth of spleen cells of an allotype-suppressed adult rabbit. The chimeric state manifested itself by the simultaneous display of light and heavy chain markers directed by donor and recipient genotypes in serum immunoglobulins (Ig), on blood lymphocytes, and on antibody molecules made in response to stimulation with three test antigens. Although the genotypes of two of the chimeras were found to be a1a1/b9b9 and the third to be a1a1/b6b9, phenotypically all three were a1a2/b5b6b9. By all of the criteria used to analyze the distribution of allotypes, Ig with the recipients' own allotypes predominated over that controlled by the donor's genotype. With only minor deviations the same proportions of recipient to donor types prevailed in total serum Ig and in the antibody fractions tested.