The affinity filtration technique using histidine as the pseudobiospecific ligand immobilized on poly(ethylene vinyl alcohol) hollow fiber membranes (Histidine-PEVA) was used to remove endotoxin (ET) from snake antivenom serum solutions. Immobilized histidine bound to horse antibodies (F(ab')(2) fragments) and ET. The effects of solution conditions on the efficiency of ET removal and protein recovery were studied. The lowest antibody adsorption capacity was obtained with acetate pH 6.0 and phosphate pH 6.5 buffers. Under both conditions, ET was removed but the highest efficiency was found for the acetate buffer. This buffer was used to determine the optimal combination of ET concentration and Q(F)/Q(i) (filtrate flow rate/inlet flow rate) ratio for achieving a high rate of ET removal without a significant loss of antibodies. Removal of 93% of the ET and recuperation of 95% of the antibodies were obtained at a Q(F)/Q(i) ratio equal to 0.68 and an initial ET concentration of 675 EU/ml, A high clearance rate of 99% was obtained with initial concentrations of 65 and 351 EU/ml for lachesis-bothrops and bothrops antivenom sera, respectively, and an almost complete depyrogenation of bothrops antivenom serum was achieved, since the final ET content in solution was 0.8 EU/mL.