Previous studies have shown that the development in rats of aflatoxin B1 (AFB1)-induced gamma-glutamyl transpeptidase-positive (GGT+) foci, indicators of early preneoplastic liver lesions, was markedly greater when a 20% casein diet was fed than when a 5% casein diet was fed during the postinitiation period. In the present study, the dose-response relationship between dietary protein level (dose) and emergence of AFB1-induced GGT+ foci (response) in livers of rats was determined. Male Fischer-344 rats fed a 20% casein diet were orally administered AFB1 at a dose level of 250 micrograms/(kg X d) (10 doses over 12 d). One week after the last dose, the animals were divided into eight groups and fed isoenergetic diets containing either 4, 6, 8, 10, 12, 15, 20 or 30% dietary casein for the remaining 12 wk of the study. The development of GGT+ foci, as measured by number and percent of liver volume occupied, displayed a response with three discrete phases. The lowest dietary protein levels, 4, 6, 8 and 10% casein, were associated with a minimal level of GGT+ foci development. Between 10 and 12% dietary casein, the development of GGT+ foci sharply increased, up to the 15-30% dietary casein level. The sudden increase in the formation of GGT+ foci at 10-12% dietary casein was just above the level of dietary casein (6-8%) required for maximum body weight gain. These results in this animal model suggest that protein intake in excess of that required to sustain maximum growth rate may enhance AFB1-induced cancer development.