A new paradigm was constructed using rats as trained animals in order to examine changes appearing in the scent of mice after a fully allogeneic bone marrow transplantation (BMT). Two rats (CAP), deprived of water 20 hours before each training session, were trained in an olfactometer to discriminate two fully allogeneic mice strains (C3H, C57) via their urine odors. Reinforcement for identification of the St was provided by a drop of water. After discrimination was built-up, the reinforcement schedule was reduced stepwise in defined blocks of trials, in order to allow transfer-of-training tests, to which urine samples of allogeneic reconstituted BMT-chimeras (C57----C3H and C3H----C57) were submitted. It was confirmed that the strain-specific urine odors of the recipients were changed by a fully allogeneic BMT. The results also showed that the urine odors of allogeneic reconstituted chimeras differed from the specific urine odor of the donor strain.