Discocotyle sagittata oncomiracidia were rapidly killed when incubated in naïve plasma and immune sera from both rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta), the killing proceeding at a faster rate with blood material from the latter fish species. The lethal activity of naïve plasma and immune sera was comparable. This was abolished after incubation at 45 degrees C for 30 min and by the addition of EDTA but not EGTA supplemented with Mg(2+), indicating that complement acting via the alternative pathway is responsible for the parasiticidal effect observed. Scanning electron micrographs showed varying degrees of surface disruption in larvae exposed to fish plasma, suggesting that complement acts by breaching the oncomiracidial tegument. Control (untreated) oncomiracidia showed no damage. Ultrastructural damage was more extensive in oncomiracidia exposed to brown trout plasma than to rainbow trout plasma for equal periods, suggesting that the complement cascade may be involved in mediating host susceptibility.