Two types of murine marrow adherent cells derived from Dexter cultures have been characterized. Exposure of C57B1/6J, ICR, or BDF1 mice to 1000 R x-ray 24 h prior to killing and establishment of liquid marrow cultures resulted in the growth of two types of adherent cells. A macrophage-like cell was phagocytic, nonspecific esterase, and acid phosphatase, positive and alkaline phosphatase, myeloperoxidase, and factor-VIII negative. The second cell was large and epithelioid in appearance, had a subpopulation of giant fat cells, was nonphagocytic, alkaline phosphatase positive, and negative for acid phosphatase, nonspecific esterase, myeloperoxidase, and factor VIII. At low inoculum levels these cells formed three types of colonies within 1-3 weeks--macrophage, epithelioid, and mixed--while at higher inoculum levels they formed confluent monolayers. These radioresistant cells supported myeloid pluripotent stem cells (CFU-S) and granulocyte-macrophage stem cells (GM-CFU-C) in liquid culture of long term nonadherent marrow cells and stimulated GM-CFU-C in agar over-lays. Refeeding liquid cultures with nonadherent cells from long-term Dexter cultures revealed that myeloperoxidase-positive cells adhered predominantly to the colonies containing epithelioid cells.