The available positive, long-term bioassays in the rat (Kimbrough et al. 1975, Norback
and Weltman 1985, Schaeffer et al. 1984) have all been performed with one dose level,
100 ppm of either Aroclor 1260 or Clophen A60, roughly corresponding to 5 mg/kg b.w.
and day. An increased frequency of liver tumours is reported in several strains. One study
covering several dose levels (25, 50, 100 ppm) of Aroclor 1254 (NCI 1978) was found to
be negative in a recent reevaluation of the pathology (IEHR 1991).