It was found that cells of Nocardia asteroides GUH-2 (virulent) were approximately 10 times more virulent than cells of N. asteroides 14759 (intermediate) and greater that 500 times more virulent than N. asteroides 10905 (avirulent) cells when early-stationary-phase cultures suspended in saline were injected intravenously into "normal" mice. There appeared to be a specific organ tropism for each strain. Thus, N. asteroides GUH-2 infected primarily the kidneys, N. asteroides 14759 infected the lungs and heart, and N. asteroides 10905 (in large doses) infected the lungs. Cyclophosphamide treatment of the mice 72 h prior to infection dramatically increased host susceptibility to nocardial infection, especially to N. asteroides 14759. However, cyclophosphamide treatment did not significantly alter the organ specificity for each strain. Cyclophosphamide greatly enhanced the ability of the nocardial strain to grow within its target organ and significantly altered normal host clearance from these organs.