This study evaluated the effectiveness of four disinfectants for irreversible hydrocolloid impressions. Impressions were made of a sterile metal model of the maxillary arch that had been contaminated with one of the following bacteria: Staphylococcus aureus, Salmonella choleraesuis, Pseudomonas aeruginosa, Mycobacterium bovis, or Bacillus subtilis. The impressions were cultured before and after immersion in one of the following disinfectants: lodofive, OMC II, 0.525% sodium hypochlorite, or Alcide LD. Alcide LD achieved a 4-log10 (99.99%) or greater reduction in colony forming units for all five organisms plus mixed oral flora. Sodium hypochlorite achieved a 4-log10 reduction in three of the five organisms and mixed oral flora. Iodofive and OMC II were ineffective against all test organisms and mixed oral flora.