Protococcus viridis is the dominant species grown on the surfaces of external walls at every climatic regions in Japan. Soiling by this algae causes the severe problem of housing maintenance. For evaluation of the effect of the anti-algal chemicals on the prevention of soiling by P. viridis, the assay methods in the medium solutions and on the coated plates were designed, and the measuring system of the cell multiplication was developed using the light absorbance of chlorophyll. Effects of chemicals on control of the algal-cell multiplication in the medium solutions were evaluated by measuring the absorbance of transmitted light through cell suspensions of P. uiridis. After 2 weeks of incubation, remarkable difference were observed depending on the types of the chemicals and its concentrations. Effects of coating by chemicals on control of the surface soiling by the algae were evaluated by decreasing of reflected light from the plates. Algal-cell densities after incubation for two weeks on the plates coated by chemicals, which had then leached in water for 4 weeks, showed good agreement with the ranks of the surface soiling on the boards after accelated out-door exposure for 5 months.

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