Summary
By studying the immunological behaviour of 50-day-old homologous chimaeras it has been established that some alteration has occurred in the donor marrow as a result of its persistence in the irradiated host.
In an attempt to understand the nature of this alteration, bone marrow from 50-day-old homologous chimaeras was serially passaged in new hosts of both host and donor strains, previously irradiated with an x-ray dose of LD99. The efficiency of the transferred marrow in protecting the secondary and tertiary hosts is compared, and the results are discussed in relation to tolerance, reversion and changes in antigenicity.