Kupffer cells (KC), by virtue of their ability to present antigen (AP) and express major histocompatibility complex (MHC) class II antigen (Ia), play a pivotal role in the host defence system against invading micro-organisms. Although haemorrhagic shock depresses the above KC functions, it is not known whether increased KC tumour necrosis factor (TNF) production and elevated TNF plasma levels following haemorrhage are responsible for it. To study this, C3H/HeN mice were pretreated intraperitoneally with either anti-murine TNF antibody (anti-TNF Ab) or saline. Twenty hours later mice were bled and maintained at a mean blood pressure of 35 mmHg for 60 min followed by adequate fluid resuscitation. Two and 24 hr later, plasma was collected and KC were isolated. AP was measured by co-culturing KC with the D10.G4.1 T(h) cell clone. Ia expression was determined by direct immunofluorescence. Interleukin (IL)-1, IL-6 and TNF levels in KC supernatants and plasma were measured with bioassays or ELISA. Haemorrhage increased circulating TNF levels by 215% at 2 hr and by 76% at 24 hr (P < 0.05), which was prevented by pretreatment with anti-TNF Ab. Haemorrhage-induced increase of circulating IL-6 was abolished (P < 0.05) at 2 hr but not at 24 hr in the anti-TNF Ab group. The suppression of KC AP (P < 0.05) and Ia expression (P < 0.05) due to haemorrhage was attenuated (P < 0.05) in anti-TNF Ab-treated mice at 2 and 24 hr and KC IL-1 and TNF synthesis was further (P < 0.01) increased. These results indicate that TNF plays a critical role in the initiation and regulation of KC AP, Ia expression, and cytokine production following haemorrhage.