Mass distribution of different infectious diseases appears even more likely in dentistry than other medical specialities. The purpose of the study is to examine different impressions with alginate materials in order to demonstrate the contamination with different pathogens. Material and method. 63 patients were included in this study. They required specific prosthetic treatment. Preliminary impression were obtained with irreversible hydrocolloid material and microbiologically tested shorthy after removal from the mouth, after washed throughly with cold water and after decontamination with specific desinfectants. Results. Microbiologic records on the alginate impressions in different stages, revealed a reduction in number of bacteria, but not completely absence, regardless of the type of decontamination substance. Conclusions. The presence of different pathogens on the surface of irreversible hydrocolloid impresions is certain. There are residual bacteria which differ from one substance to another, irrespective of cleaning stages and decontamination materials used in daily manipulation of dental impresion. Even if time consuming, decontamination of alginate impression is compulsory in dentistry.