Bovine serum albumin (BSA) associated with thymus cells from a donor differing for 2 or 3 allotypic markers (a, b, A14–15) from recipient animals was immunogenic when administered to newborn rabbits. The same dose (15 μg)of antigen, as well as larger doses, administered in solution were usually tolerogenic. Recipients of thymus cells develop detectable levels of donor type immunoglobulin markers persisting for 8–10 weeks. The anti-BSA appeared to be exclusively of host origin; no relationship existed between the presence of the donor type immunoglobulin and the antibody response. Cellular chimeras in the peripheral blood were not demonstrated using sex chromosomes as markers.