Summary A modified XC assay for murine leukemia virus (MuLV) employing splenocytes taken directly from the animal is described. This modification can be more than 1000 times more sensitive than XC plaque assays employing tissue extracts. This technique should lend itself readily to the quantitation of infectious MuLV in defined populations of lymphoid cells. This research is supported by USPHS Grants CA 10018 and AM 07937, and a Grant from the Damon Runyon Memorial Fund for Cancer Research.