Because of the increased production of carbon monoxide from man's activities, it was considered important to study the biosphere's sink for it. The report describes studies conducted to determine if soils and vegetation could serve as a sink for atmospheric carbon monoxide. This was accomplished by exposing various soils and plant samples to experimental atmospheres containing carbon monoxide. The results show that higher plants tested had no detectable capacity for decreasing carbon monoxide, soils from a wide range of ecotypes depleted carbon monoxide, prolonged exposure of soil to carbon monoxide reduced the soil's capacity to deplete carbon monoxide, soils oxidize carbon monoxide to carbon dioxide, and carbon monoxide depleting microorganisms were identified.