Abstract
A quantitative study has been made of the interactions between radiation leukemia virus (RadLV), its murine sarcoma virus pseudotype, and their C57BL host cells. The elimination of interference phenomena by delayed infection of cells with RadLV made possible the quantitative determination of the pseudotype in terms of defective sarcoma and endogenous RadLV particles. This in turn permitted the quantitative assessment of RadLV helper activity and of the various factors which influence the accuracy and sensitivity of the helper assay.