The recovery of phenytoin from mixtures containing different phenytoin formulations and protein mixtures was studied. Three phenytoin solutions (40 mg/microL) were prepared, each in triplicate, from phenytoin tablets, phenytoin suspension, and bulk phenytoin powder. These solutions were mixed with equivalent volumes of two commercially available feeding formulas (Replete and Ultracal) and two isolated protein mixtures (casein protein mixture and why protein isolates mixture) and placed in ultrafiltration tubes. The mixtures were centrifuged, and phenytoin recovery was determined by using high-performance liquid chromatography. Control data were also obtained before and after the experiment. There was no difference in phenytoin recovery when comparing phenytoin tablets versus phenytoin suspension in any of the protein media. There was a significant difference in phenytoin recovery when comparing the standard phenytoin solution mixed with Replete (32.51%) versus Ultracal (37.71%). There was also a significant difference in recovery when comparing the standard solution mixed with the calcium caseinate mixture (48.41%) versus the whey protein isolates mixture (82.01%). While the difference in recovery between Replete and Ultracal was expected, the significantly higher recovery of phenytoin from the whey protein mixture versus the calcium caseinate mixture indicated a much lower binding affinity between phenytoin and whey protein than with phenytoin and casein. The recovery of unbound phenytoin from feeding formulas and solutions of protein isolates did not differ with phenytoin formulations. Ultracal had a lower level of binding to phenytoin than Replete; whey protein had a lower level of binding than casein.