Impression materials or prostheses can be contaminated with oral microflora and provide a significant source for cross-contamination. A study of such contamination was carried out using an approach different from that of infection control, which has often been investigated in previous studies. The study focused on microorganisms known to cause local and systemic diseases and which are normally found in the oral flora. The persistence of Streptococcus mutans (S. mutans), Escherichia coli (E. coli), Staphylococcus aureus and Candida albicans (C. albicans) on zinc-oxide eugenol, silicone rubber, irreversible hydrocolloid and polyether-rubber was investigated using 99mTc-labelled microorganisms. Ten specimens from each of the four impression materials were prepared as discs of 3 mm in height and 10 mm in diameter. After the specimens had been placed into a suspension of 99mTc-labelled microorganisms, remaining radioactivity was counted in a gamma counter. According to own findings, S. mutans was the most, and E. coli the least persistent on the specimen surfaces. The number of microorganisms removed after washing was less than the amount remaining on the surfaces. C. albicans was removed most easily from all impression surfaces that bore persistent microorganisms after washing. Other microorganisms showed various degrees of persistence according to the impression material.