The effects of high and low dietary protein intervention on the growth and development of hepatic preneoplastic lesions were examined. Inbred male F344 rats fed a semipurified (AIN-76) 20% casein diet were given doses orally of aflatoxin B1 (10 doses, each 250 micrograms/kg). One week after the last dose, animals were fed diets containing either 5 or 20% casein. Animals fed a 5% casein diet throughout the 12-week postdosing period had a marked reduction in development of gamma-glutamyltransferase-positive foci. Animals fed a 20% casein diet throughout the same period had the highest response. Groups fed the 5% casein diet for half of the postdosing period and 20% for the other half had intermediate responses. The data show that the high response observed in animals fed a high-protein diet can be inhibited by the postinitiation intervention of a low-protein diet. Likewise, the low response observed in the animals fed a low-protein diet was increased by the introduction of a high-protein diet late in the experiment.