Liver cells, isolated from male Sprague-Dawley rats and established in primary culture, were treated with a dose of 180 pmol aflatoxin B1 (AFB1) for 2 hr. Reticuloendothelial nonparenchymal cells (NPC) metabolized a total of 28% of the AFB1. In contrast, parenchymal epithelial cells or hepatocytes (HC) were highly efficient in conjugating AFB1 to water-soluble products (50-70% of the dose) but also bound a large proportion (27%) of the toxin to cellular macromolecules. In comparison to NPC, HC bound 90-fold more AFB1 per cell. Cytotoxicity in primary cultures was evaluated by changes in membrane integrity (lactate dehydrogenase release), inhibition of glutathione-S-transferase activity and cell monolayer morphology over a range of AFB1 doses. No response was detected in NPC, whereas HC exhibited dose-related cytotoxic responses to AFB1. In rat liver both cell types form AFB1-DNA adducts in a dose-dependent fashion and yet only epithelial cell carcinomas have been observed in whole animal studies. The possibility that cell-specific AFB1 cytotoxicity produces a promotional stimulus for hepatocellular carcinoma is discussed.