Retroviral vectors have provided a means for the introduction of functioning exogenous genes into the hematopoietic system of whole animals. Although these vectors are quite efficient in the mouse model, when applied to non-murine in vivo systems, the efficiency of gene transfer has diminished to impractical levels. Since in vivo analyses are expensive and time consuming, in vitro models have been developed to speed the evaluation of alternative protocols. Using in vitro colony assays, three approaches were evaluated for their ability to improve the infectivity of hematopoietic progenitor cells with retroviral vectors. Exogenously applied hematopoietic growth factors increased the proportion of hematopoietic colonies in vitro up to an average of 5 fold. When alternative sources of progenitors, such as fetal cord blood, were used, improvements in infection efficiency were also obtained. Finally, evidence was acquired suggesting that xenotropic packaging of vectors also improved infection efficiency.