Dextran interference in biuret-type assays of total serum proteins was investigated in a Belgian National External Quality Assurance Survey with 256 participants. In vitro supplementation of therapeutic (10% Gentran 70) dextran concentrations showed a broadly varying (from 0 to 20%) negative interference. The analytical interference was found to depend on both the sodium hydroxide and tartrate concentrations in the reagent formulation. The dry chemistry biuret method was not affected by the dextran interference. In a number of cases, the effects observed may be of clinical importance. Both clinicians and laboratory staff should be aware of the persistence of this analytical problem