In normal young lambs the bone marrow was selectively labelled with fluorescein isothiocyanate by a temporary perfusion of one hind-leg. One day later, the incidence of bone marrow emigrants in different lymph nodes, spleen, Peyer's patches, thymus, non-perfused bone marrow and blood was determined. The emigrants were also phenotyped by the use of monoclonal antibodies and classified into monocytes or lymphocyte subsets. Large numbers of lymphocytes left the bone marrow of the perfused leg during 1 day. Considerable numbers of cells migrated to other bone marrow compartments. Varying numbers of mononuclear emigrants were found in peripheral lymphoid organs, with labelling indices ranging from 1.06% in the blood to 0.004% in the thymus. In the spleen, comparable numbers of B- and T-lymphocyte emigrants from the bone marrow were found, whereas in the blood, lymph nodes and jejunal Peyer's patches many more emigrants were T lymphocytes than B lymphocytes. In the prescapular lymph nodes, for instance, 90.4% of emigrants were T cells but only 9.6% were B cells. Based on the large numbers of lymphocytes emigrating from the bone marrow, their phenotypes and their entry into other bone marrow compartments, it it can be concluded that the bone marrow of young lambs is an integral part of the migratory route of lymphocytes.