Research into the pathogenesis of acetaminophen (paracetamol)-induced hepatotoxicity has concentrated on the generation of toxic metabolites by the hepatocytes. It has, however, recently been shown that human macrophages cultured with acetaminophen secrete increased quantities of tumour necrosis factor (TNF). This study examines whether macrophages have a direct role in acetaminophen toxicity, using a mouse model in which it is possible to eliminate more that 99 per cent of hepatic macrophages by previously injecting liposomes containing dichloromethylene disphosphonate (DMDP). Acetaminophen-induced liver damage was assessed biochemically and histologically. It was shown that the liver damage occurring 0.5, 1, and 2 h after an intraperitoneal injection of acetaminophen was significantly less in mice previously injected with liposomes containing DMDP than in previously untreated mice, or mice previously injected with empty liposomes. By 4 h there was no difference between the groups. We conclude that macrophages play an early and probably a direct role in mediating the liver damage due to acetaminophen. This is consistent with the role that macrophages have been shown to play in the pathogenesis of alcohol-induced liver damage.