The validity of a new technique for studying drug protein binding is tested. By this gel diffusion or batch method using dry polyacrylamide, sulfadiazine and sulfafurazole are bound by human plasma proteins to the same extent as reported by other authors with different techniques. The binding of warfarin to fresh human plasma was quantitatively less than expected. This might be due to the possibility that other plasma constituents of non-fasted test persons displace warfarin from protein binding sites, since it is bound more in buffer solutions containing physiological concentrations of human albumin. Phenylbutazone displaced warfarin from its binding sites in diluted human serum albumin concentrations. Quinidine was found to be adsorbed by the otherwise inert polyacrylamide gel and the method is therefore not applicable to quinidine binding studies.