Aflatoxin B1-induced liver lesion development is readily modified by dietary protein intake. Earlier work had shown that low-protein diets enhanced the acutely toxic lesion but depressed the carcinogenic lesion. This study examined the emergence of these lesions as a function of dietary protein intake, particularly with respect to whether the protein modification occurred during or after the aflatoxin B12 dosing period. High (20%) and low (5%) casein diets were fed to growing Fischer 344 rats during the dosing and postdosing periods of aflatoxin B2-induced hepatic preneoplastic lesion development. Focal areas of hepatocellular alteration were identified and quantitated by staining sections of liver for gamma-glutamyl transferase (GGT). Animals fed low casein diets during the dosing period displayed a characteristic spectrum of lesions including hepatomegaly, severe bile duct proliferation, cholangiofibrosis, and a tendency for developing large remodeling GGT-positive foci. These lesions were regarded as symptomatic of acute hepatoxicity. Animals fed high-protein diets during the dosing period had small, densely stained, GGT-positive foci, with only mild bile duct proliferation and no cholangiofibrosis, hepatomegaly, or large, remodeling GGT-positive foci. During the postdosing period, protein modulation markedly influenced the total number of foci. Animals fed high casein diets during this period exhibited an approximate 6-fold increase in the number of foci, regardless of the level of protein fed during the earlier dosing period. The marked increase in foci number (as well as area of liver occupied) in high casein diet animals during the postdosing period is regarded as an increased tendency for tumor development.