Procedure is described for purifying low-molecular-weight factors with antigen-aspecific properties from a dialysate of human leukocyte extract. It includes gel chromatography on Sephadex G-25 and G-15, ion-exchange chromatography, reversed-phase high-performance liquid chromatography (HPLC) on a C18 hydrophobic column and gel permeation HPLC. The immunosuppressive factor (mol.wt. 800-1000) was purified to near homogeneity. It is probably of peptidic nature, although it is pronase resistant. The enhancer factor (mol.wt. 300-600) is eluted from chromatographic columns together with a hypoxanthine-like substance. Nevertheless, the biological activity cannot be attributed to the purine derivative. Identification of this amplifier activity is still lacking.
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