The infectivity of vectors incorporating SCFXA chimeric envelopes is increased by the addition of exogenous rHuSCF. Infection of two different Kit-positive cells lines was performed in the presence of 100 nmol/L SCF. There was no change in the titer of wild-type 4070A vectors, but the titer of the vectors incorporating SCFXA chimeric envelopes increased fivefold on HMC-1 cells and 11-fold on TF-1 cells. The plain bars represent the background level of infectivity and the striped bars represent the fold increase in titer on addition of SCF.