The possibility that selection of an initiating agent could have a sig
nificant impact on the ability to detect subsequent promoting activity
of peroxisome proliferators was examined, Initiation was achieved by
established methods using 2-acetylaminofluorene (2-AAF: 0.02% in diet
for 8 weeks) or diethylnitrosamine (DEN; 150 mg/kg body wt by single i
.p. injection) in male F344 rats. Following initiation, the peroxisome
proliferators WY-14,643 or clofibrate were each fed (0.1% of diet) fo
r up to 37 weeks. Both WY-14,643 and clofibrate lacked promoting activ
ity, as measured by increases in the volume density of homogeneous bas
ophilic foci and incidence or multiplicity of hepatocellular neoplasia
following 2-AAF initiation compared to non-initiated controls. These
negative results sharply contrasted with the observed promoting activi
ty of dietary WY-14,643 and clofibrate following DEN initiation. Perox
isome proliferation, measured as induction of acyl-CoA oxidase activit
y, was consistently observed in peroxisome proliferator-fed rats despi
te prior initiation with 2-AAF or DEN. These results suggest that dete
ction of promoting activity for peroxisome proliferators depends on se
lection of the initiating agent.