Using adapted retroviruses for gene delivery is a modern and powerful tool in biological research as well as a promising approach for gene therapy. An important limitation for the extensive use of retroviral vectors is the low infection rate in target cells such as pulmonary vascular endothelial cells due to the insufficient infectivity of standard retrovirus supernatants that can only be overcome by complicated methods of virus concentration. This paper describes two easy methods to augment target cell infectivity, first by increasing the retroviral titer in the medium collected from packaging cells by stimulation of viral propagation with dexamethasone, and second, by increasing target cell sensitivity to retroviral infection by the glucocorticoid receptor antagonist, mifepristone. Using this method, we increased the infectivity of pulmonary microvascular endothelial cells from 16% to 85%. We demonstrate that mifepristone increased the susceptibility of target cells to retroviruses without increasing the viral titer of the supernatant. Dexamethasone, but not mifepristone, increased expression of delivered proteins such as GFP that are important for early identification of infected cells. Each, or both step(s), may be included in a standard protocol for retrovirus propagation and infection of target cells.