Triton X-114 and cation-exchange chromatography, SP-Sepharose FF, removed endotoxins from solutions containing recombinant histone H1.5. Dissociated endotoxins were removed but fractions containing histone H1.5 were enhanced in the elution step. The final concentration of endotoxins, measured by a limulus amoebocyte lysate (LAL) assay, was below 0.05 EU mg(-1) histone H1.5. The recovery of protein was above 95%.