In 23 dwellings in Copenhagen high concentrations of airborne mould spores were shown by the "open Petri dish" method. The families were instructed how to reduce this contamination. One year later the number of mould spores was estimated again. At that time their number was 1/10 of that shown at the first sampling in 11 dwellings, and in 7 dwellings the reduction of mould spores decreased to 1/5. Only one dwelling showed no reduction.