A method for reducing endotoxin contamination in various solutions by immobilized histidine is described. Immobilized histidine is a porous adsorbent suitable for the adsorption of endotoxin with a high affinity over a wide range of pH and temperature and at low ionic strength (gamma/2 less than or equal to 0.1). When a purified endotoxin originating from Escherichia coli UKT-B was studied, the apparent dissociation constant between endotoxin and the adsorbent was 7.3 X 10(-13) M. The adsorbent was able to remove various kinds of endotoxin originating from gram-negative bacteria; the concentration of endotoxin was reduced from 1000 to less than 0.01 ng/ml in water. It is shown that the adsorbent specifically adsorbs endotoxin provided that the adsorption conditions are properly selected. Some examples of the specific removal of endotoxin from high-molecular-weight physiologically active substances such as tumor necrosis factor and lysozyme are shown.