The colonization of exterior masonry paints by algae and lichens causes disfigurement of the paint surface and possible disruption of the paint film. Local environmental factors and the nature of the substrate can be as important as general climate in determining the occurrence and severity of fouling. Control of algae and lichens can be achieved by removing existing growth by mechanical means and the use of an algicidal wash, and by the use of a paint algicide to prevent surface contamination. Most commercial paint film fungicides are not effective algicides. No official standard test methods exist for determining the effectiveness of algicidal washes or paint algicides. A new vermiculite bed method for testing algicides for washes appears promising, and the algal test cabinet developed by the Paint Research Association in the United Kingdom has proved successful for the laboratory testing of paint algicides. The International Biodeterioration Research Group is involved in developing a laboratory test method for algicidal paints.