The article deals with comparative study of antimicrobial efficiency of metallurgical slags suitable for construction applications. The tested slags were as follows: granulated blast-furnace slag (GS1), air cooled blast-furnace slag (AS2), demetallized steel slag (DS3), calcerous ladle slag (LS4), slag from copper refining (CS5). The antimicrobial activity was tested on selected representatives of bacteria, yeasts, and filamentous fungi. The highest antibacterial activity possessed LS4, which intensely inhibited growth of bacteria with the lowest concentration of slag (10%) in the growth media. 100% inhibition of growth of some bacteria was observed only in slags LS4, DS3 and AS2 in concentrations 20% - 60% of slag. Antibacterial activity of slag samples was decreasing in the order: LS4 > DS3 > AS2 > GS1 > CS5. Growth of all model yeasts was 100% inhibited at as low concentration as 20% of slag GS1 and DS3, and 10% of slag LS4. Antiyeast activity of slags was decreasing in the order: LS4 > GS1 = DS3 > AS2 > CS5. Regarding that filamentous fungi were selectively sensitive to presence of slags, it is possible to determine only approximate order of inhibition effectiveness of slags to fungi: LS4 > GS1 = DS3 > AS2 = CS5.