In this in-vivo study, dialysate and serum endotoxin was evaluated before and after haemodialysis with small-pore (PS400) and large-pore (PS600) polysulphone dialysers, and before and after haemodiafiltration with the PS600 filter. The source of the endotoxin was the presence in dialysate of Pseudomonads at a concentration of 103–104 CFU/ml. Endotoxin was measured by a modified chromogenic limulus amoebocyte lysate (LAL) assay. In spite of dialysate endotoxin concentrations > 100 pg/ml, no changes in pre- versus posttreatment LAL reactivity were observed in PS400 dialysis and PS600 haemodiafiltration. In contrast, PS600 haemodialysis was related to an increase in serum LAL reactivity from 1.3 ± 1.5 to 3.8 ± 2.0 pg/ml (n=15, 1in-vivo endotoxin transfer during haemodialysis with small-pore dialyser membranes, and during haemodiafiltration with membranes with larger pores. An increase in LAL reactivity during haemodialysis with membranes with larger pores is, however, present, presumably due to the occurrence of backdiffusion/filtration with that specific strategy.