Summary Sensitivity of three assay techniques for the detection of murine leukemia viruses were compared. These techniques were the mouse antibody production test, appearance of new surface antigen(s) on JLS-V9 cells, and production of plaques on D56 cells. The sensitivity of the three assays in the detection of murine leukemia virus is similar. The plaque assay on D56 cells offers certain advantages over the other tests since smaller quantities of leukemia virus could be detected on day 5 after infection by using this technique. The antibody production test requires 9 weeks for completion. For viruses with a differing host range which will induce neither plaque nor membrane antigens on D56 cells, the assay of JS-V9 cells may be the preferred method. The authors thank Dr. Robert H. Bassin, Viral Leukemia and Lymphoma Branch, NCI, Bethesda, MD, for providing the D56 cell line. These investigations were conducted under Contract No. NIH-69-2005 within the special Virus Cancer Program of the National Cancer Institute, National Institutes of Health, U.S. Public Health Service; the Jane Coffin Childs Memorial Fund for Medical Research, Project No. 287; and grants from the Swedish Cancer Society. The expert technical assistance of Mrs. Britt Salven is gratefully acknowledged.