Detection of B-ecotropic murine leukemia virus (MuLV) in the spleen of mice segregating the splenomegaly phenotype. Spleen cells from control A/J (5- to 13-week-old) and BXH-2 (5- to 16-week-old) parents and from (A/J × BXH-2) F1 (20- to 36-week-old) and F2 (5- to 16-week-old) mice showing either presence (high SI; H) or absence (low SI; L) of splenomegaly were harvested and used to detect the presence of B-ecotropic virus by an XC plaque assay, as described in “Materials and methods.” BALB 3T3 mouse cells were exposed to serial dilutions of mitomycin C–treated splenocytes. After reaching confluence, the infected cultures were UV irradiated and overlaid with XC cells. Forty-eight hours later, cells were fixed and stained for syncytial plaques count (plaque-forming units, PFU), and a viral titer was calculated (log PFU).