Aylin Atilla VMD, MS, Harry W. Boothe DVM, MS, Diplomate ACVS, Melissa Tollett BS, Sue Duran BS, MS, PhD, Dubraska Campos Diaz DVM, MS, Jameson Sofge BS, Dawn M. Boothe DVM, PhD, Diplomate ACVIM & ACVCP Veterinary Surgery August 2010 39 6 715-21 Objective: To describe in vitro elution characteristics of amikacin and vancomycin from calcium sulfate hemihydrate 98% (plaster of Paris, POP) beads and characterize eluent inhibition of Staphylococcus spp. Study Design: Experimental study. Methods: POP beads were impregnated with amikacin or vancomycin alone or in combination and then incubated alone or in combination for 84 days at 37°C in plastic tubes containing sterile phosphate-buffered saline (PBS). Beads containing no antimicrobial served as negative control. Beads were intermittently moved to a new tube containing drug-free PBS. Antimicrobial was measured in the eluent using a polarized fluorescent immunoassay. Eluent inhibition of Staphylococcus spp. was determined at each time point. Results: Antimicrobial release from beads was characterized by an initial rapid phase then a slower phase. Although antimicrobial release from beads occurred throughout the 84 days, most was in the first 24 hours, except for vancomycin alone. Duration of eluent inhibition of Staphylococcus spp. growth ranged from 0.5 (amikacin alone) to 56 days (vancomycin alone). Control eluent did not inhibit bacterial growth. Conclusions: Amikacin elution from POP beads was rapid, inhibiting growth for <24 hours with or without vancomycin. Vancomycin elution was slower and inhibited growth for 56 days alone or for 5 days with amikacin. Clinical Relevance: Vancomycin-impregnated beads appear to be reasonable as a therapeutic option whereas amikacin-impregnated POP beads and amikacin and vancomycin combinations may require further study before considering as a therapeutic option.