Biological and immunological characteristics of the reticuloendothelial system of irradiation bone marrow chimeric mice and macrophages collected from various tissue sources of the mice were studied. The chimeras showed comparable activities in carbon clearance to those of normal donor or recipient mice. The macrophages from spleen, lymph node, bone marrow, peripheral blood, liver, peritoneal cavity, and lung were demonstrated to be of donor marrow origin. They showed almost the same enzyme activities and phagocytic capability of sheep erythrocytes (SRBC, E), SRBC sensitized with anti-SRBC IgG (EA), and SRBC sensitized with anti-SRBC IgM and coated with complement (EAC) as those of normal mice. Proportions of Fc receptor and complement receptor-positive cells are also in normal range. In addition, the antigen-presenting capability of the chimeric macrophages for in vitro primary antibody response to SRBC was intact. These observations suggest that the reticuloendothelial system and macrophages of allogeneic bone marrow chimeras where donor and recipient differ at the major histocompatibility complex have no defect so far as could be ascertained by the present study.