Plasma from 33 patients at risk of multiple organ failure (MOF) after major trauma was tested for a priming effect on neutrophils, and for the presence of platelet-activating factor (PAF) activity and interleukin (IL) 8. Plasma sampled at 3, 6, 12 and 24 h after injury significantly primed normal neutrophils to release mean(s.e.m.) 1·26(0·19), 1·33(0·26), 1·04(0·14) and 0·86(0·13) nmol superoxide per min per 1·3 × 106 neutrophils respectively (P<0·05). Priming at 3 h after injury was inhibited by mean(s.e.m.) 63·8(7·0) per cent by the PAF antagonist, WEB 2170 (P<0·01). Mean(s.e.m.) plasma IL-8 was raised at 6 and 12 h after injury to 785(183) and 836(175) pg/ml (P<0·01). At 12 h after injury the plasma IL-8 level correlated directly with the number of units of red blood cells transfused (r=0·64, P<0·01), and was significantly higher in the group of six patients who developed MOF (P<0·05). These data suggest that after trauma the mediators PAF and IL-8 appear sequentially in the circulation, are potential mechanisms of circulating neutrophil priming, and that IL-8 may also be an early biochemical marker predicting the onset of MOF.