Peptides were isolated from the middle molecule fraction of hemofiltrate obtained from uremic patients by adsorption of the organic compounds on RP 18. By stepwise elution an enriched peptide fraction was obtained. This procedure was followed by a combination of ion-exchange-chromatography, gel-chromatography and HPLC. The pure peptides obtained in this way were investigated by liquid secondary ion mass spectrometry (LSIMS)-if necessary after enzymatic digestion. The identified peptides turned out to be fragments of the human serum albumin and beta 2-microglobulin.