Plasma from patients with renal failure has decreased binding of many drugs. The decreased binding of anionic drugs is pronounced and is apparently due to occupation of protein binding sites by non-dialysable endogenous acidic compounds. Organic bases seem to bind normally, particularly if hypoproteinaemia or hypoalbuminaemia are not present. This modification of drug binding to plasma proteins from uraemic patients must be considered when plasma concentrations of acidic drugs are measured.