Emission of heavy metals during industrial tests of co-combustion of alternative fuel added to hard coal (10%) in a boiler with mech. grate was studied. The emission stds. for heavy metals were slightly exceeded. Both slag and fly ash from the co-combustion contained together larger quantities of heavy metals when compared to hard coal combustion. Pb, Sb, Co, As, Cd and Hg showed similar behavior in coal combustion as well as in co-combustion of coal with the alternative fuel. Cd and In contents in fly ash from co-combustion were higher than the allowed by resp. stds.