Five microorganisms--Escherichia coli, Streptococcus mutans, Staphylococcus aureus, Serratia marcescens, and Bacillus subtilis--were placed into the canine, first premolar, first molar, and second molar of sterilized quadrant addition-reaction silicone impressions. Four disinfectant solutions--iodophor, neutral glutaraldehyde, phenol, and sodium hypochlorite--were spatulated with sterilized Type IV stone and vibrated into the intentionally contaminated impressions. The casts were removed at 1 and 24 hours, and the surfaces were swabbed, plated on BHI agar, and incubated at 37 degrees C for 3 days. Colony forming units were counted. At 1 hour, only sodium hypochlorite and glutaraldehyde effectively reduced the number of bacteria to the negative control. Incorporation of sodium hypochlorite, glutaraldehyde, and iodophor into gypsum casts reduced viable bacteria to a level equal to the negative controls at 24 hours.