Thiabendazole (TBZ) is used as an anthelminthic for domestic animals and as a pre- or post harvest fungicide for field crops and stored fruits and vegetables. TBZ is metabolized in livestock to 5-hydroxythiabendazole (5-OH-TBZ), which is metabolized further to the sulfate conjugate of 5-hydroxythiabendazole. Various multiresidue methods have been developed for the analysis of benzimidazole residues in milk, but these methods cannot be used to determine the sulfate conjugate of thiabendazole. We have used acid hydrolysis (4 hrs with a conventional oven, 1 hr. with a microwave digestion system) to convert the sulfate conjugate to 5-OH-TBZ. The extract is then subjected to a solid phase extraction cleanup, followed by HPLC determination with UV and fluorescence detection. The amount of incurred residue of TBZ, 5-OH-TBZ, and 5-OH-TBZ sulfate conjugate, in milk collected from 8-120 hrs post dose was determined. Thiabendazole residues in the milk ranged from 3500 ppb at 8 hrs. post dose (p.d.) to <1.0 ppb at 120 hrs p.d. Over 95% of the TBZ residue found in the milk was in the form of sulfate conjugate 5-OH-TBZ. We also evaluated a benzimidazole immunoassay kit for the determination of free TBZ, 5-OH-TBZ, and sulfate conjugates of 5-OH-TBZ in milk.