Abstract Objective. The aim of this study was to measure the in vitro killing effects five commercial alginate impression materials had on five test microorganisms. Methods. Two alginates with no added disinfectant, one supplemented with chlorhexidine and two others containing quaternary ammonium compounds were tested. Challenge microbes included two gram-positive cocci, two gram-negative bacilli and a yeast. Saline solutions containing standardized concentrations of test microbes were used to mix the alginates. Some set specimens were immediately homogenized and the resulting fluids diluted and spread plated. Other specimens were processed 30 or 60 min after setting. After culturing, the numbers of colonies were counted and the levels of microbial reductions determined. Results. Unsupplemented alginates had no antimicrobial effects. The quaternary-ammonium-containing alginates were completely effective against all five test microorganisms. The alginate with chlorhexidine killed all the gram-negative bacilli and the majority (95–99%) of the gram-positive cocci and yeast. Significance. Results indicated that disinfectant-containing alginate impression materials could reduce the number of soiling microorganisms present on and within test specimens.