The effect of aflatoxin B1 on the uptake and incorporation of [3H]leucine and [3H]uridine and on phagocytosis of latex particles was studied using cultures of rat liver macrophages (Kuffer cells). Aflatoxin B1 inhibited the incorporation of both isotopes, but inhibition of uridine incorporation was greater than that of leucine, suggesting that RNA synthesis was a major site of inhibition. Aflatoxin B1 also inhibited phagocytosis of latex particles in a time- and dose-dependent manner.