Plasma protein binding of diphenylhydantoin (DPH) in normal plasma was investigated with an ultrafiltration technique (at room temperature) with the use of 14C‐labeled DPH. DPH was 92.6 per cent bound. There was no marked difference between sexes or individuals. In blood donor plasma the bound fraction was 93 per cent at a concentration of DPH of 16 μg per milliliter (therapeutic concentration). The binding was less at 37° C. than at wom temperature. The effect on DPH binding of some other drugs, including various anti‐epileptics, was investigated. Of these only salicylic acid, sulfafurazole and phenylbutazone (in concentrations which may be obtained clinically) decreased DPH binding. The effects of plasma dilution and of dilution of both plasma and DPH were investigated. In the former case unbound concentrations increased but were fairly constant in the latter. The clinical significance of these findings is discussed.