Male weanling F344 rats were orally gavaged with aflatoxin B1 (AFB1) in daily doses of 200, 235, 270, 300, and 350 micrograms/kg/day for a total of 10 doses over a 12-day period, and then 1 week after the last dose they were fed diets of varying protein (casein) content to compare the contribution of AFB1 dose and dietary protein level on the development of presumptive preneoplastic gamma-glutamyltransferase-positive (GGT+) foci in rat liver. All animals were fed the same 20% dietary casein level during the dosing period. One week after the end of the dosing period, one-half of the animals in each dose group were then continued on the 20% casein diet for the entire 12-week foci-development period; the remaining half in each dose group were fed lower levels of dietary casein during the foci-development period for the increasing AFB1 dose groups (20, 16, 12, 8, and 4% casein for the 235-, 250-, 270-, 300-, and 350-micrograms/kg/day groups, respectively). The AFB1 dose groups used were determined in a preliminary experiment. In this previous experiment, a clearly discernible threshold dose at about 100-150 micrograms AFB1/kg/day (below which no GGT+ foci were observed) and a steep slope between 150 and 400 micrograms/kg/day were produced. In the second experiment, while the expected positive slope of (AFB1) dose versus (GGT+ foci) response relationship was found for animals fed the 20% casein diet, the dose response for the animals fed the lower levels of casein was eliminated, providing evidence that nutrient intake during the postdosing foci development is more rate limiting toward the development of these preneoplastic lesions than is the carcinogen dose.