The apparatus includes a dialyzer (1), and an arrangement for bringing into solution protein-bonding substances to be removed, and at least one device for dialysis, filtration or diafiltration. One dialysis fluid includes an adsorber for the substances to be removed from the biological fluid. An Independent claim is included for a method of removing unwanted substances from a biological fluid. Dialyzable compounds are added to the dialysis fluid and/or the biological fluid, and may comprise caffeine and chelating compound for metal cations, e.g. penicillamine, trientine, deferoxamine, preferiprone, HBED, vitamin C, BAL, DMPS or DMSA.
