Cyclophosphamide severely inhibits the function of various hemic precursor cells in mice. In these studies we compared the effects of Cy on early B-lymphocytic precursor cells in mouse bone marrow to those effects on bone marrow myeloid precursor cells and splenic lymphoid precursor cells. These studies demonstrated that: 1) Pre-ARC in the bone marrow were extremely sensitive to even low doses of Cy (50 mg/kg) with virtually no renewal of this precursor population 11 days after injection of 200 mg/kg. 2) The sensitivity of pre-ARC to Cy along with the lack of their renewal was exemplified by the virtual absence of functional ARC in the spleen when assessed 11 days after injection of 200 mg/kg of Cy. 3) A rapid multiphasic renewal pattern was observed for myelocytic progenitor cells. 4) In general, LPS responsive cells were suppressed to a greater extent than were Con A responsive cells, especially with lower doses of Cy.