OBJECTIVE Upregulation of neutrophil adhesion molecules (CD11b and L-selectin) and release of a modulating cytokine (IL8) have been reported in vivo and in vitro in adult cardiopulmonary bypass. The aim of this study was to determine whether paediatric bypass preparations have similar influences and whether neutrophil-endothelium interactions are required for IL8 release. METHODS In vitro paediatric cardiopulmonary bypass circuits (n = 15) were constructed (identical to those used clinically), as well as static loops (n = 15) using donor blood. The effects of circulation and temperature (17 degrees C, 25 degrees C, 37 degrees C) on the initiation of acute inflammation were examined. Cellular expressions of neutrophil adhesion molecules CD11b and L-selectin were assayed by immunofluorescence technique, and serum IL8, IL6, TNF-alpha, leucocyte elastase, and terminal complement complex were measured by ELISA. RESULTS In all experiments, an immediate increase in CD11b expression occurred [median values, in relative fluorescence units: 64.9 (range 45.3-212.9) at rest; 365.2 (205-835.4) at 10 min; P < 0.001], along with a decrease in L-selectin expression [153.5 (115.5-220.7) at rest; 42 (12-134) at 10 min; P < 0.01]. Serum concentrations of the following increased gradually and were higher in circulation than in static loops: IL8 [1500 (500-2500) pg.ml-1 in circuit v 600 (180-1500) pg.ml-1 in loop, P < 0.001]; TNF-alpha P < 0.05]; and terminal complement complex [25.9 (6.8-120) v 4.7 (0-21.6) AU.ml-1, P < 0.01]. Cooling decreased and rewarming increased upregulation of CD11b and downregulation of L-selectin and release of IL8. IL6 was undetectable. CONCLUSIONS In the absence of endothelium, in vitro paediatric cardiopulmonary bypass causes profound acute inflammatory changes in donor blood with release of IL8. These changes were greater than in adult cardiopulmonary bypass. Temperature variation and circulation modulate the responses.