Properties of flat variants of murine sarcoma virus transformed non‐producer cells isolated after high‐temperature passage

High‐temperature passage (40.5°C) increased the incidence of flat variants obtained from Kirsten sarcoma virus transformed BALB/3T3 non‐producer cells from less than 0.1% to 4%. The flat variants had growth properties similar to those of the BALB/3T3 line. The variant lines did not release detectable type C virus, but some contained intracisternal type‐A particles. Super‐infection of the variants with murine leukemia virus (MuLV) resulted in retransformation and release of sarcoma virus. Some of the variant lines may be suitable indicator cells for the assay of MuLV. Modal chromosome numbers of the variant lines were considerably higher than those of their progenitor cell line.