Tritiated thymidine-labeling data in individual and parabiotic rats showed that macrophages in peritoneal exudates were derived from cells in the blood which were the progeny of rapidly and continuously proliferating precursors. The characteristics of this population identify them with free macrophages studied in other sites; similarly, they can be obtained from transfused bone marrow.
Cells in the exudates which were morphologically indistinguishable from small lymphocytes were also found to have the labeling features of a rapidly proliferating population in contrast with the known kinetics of the majority of small lymphocytes in blood and thoracic duct lymph. However, experimental evidence indicated that the lymphocytelike exudate cells had emigrated from the blood and that bone marrow was a source of their precursors. These findings support the concept of the heterogeneity of lymphocytes.
The possible relationships among the mononuclear cells is discussed.