SUMMARYMembrane filters retain14C bicarbonate,14C glycollate, and other14C labeled substances from filtrates of algal cultures and lake water. By refiltering different volumes of the filtrate from algal cultures and from lake water after incubation with14C bicarbonate, it was shown that the labeled material retained was not proportional to volume but showed a saturation effect with increasing volume filtered. When the radioactivity retained by a filter is divided by the volume filtered, decreasing values are obtained with increased volume filtered. This radioactivity may represent a significant addition to the radioactivity in particulate material on the filters, resulting in a similar type of curve when different volumes of lake water or cultures are filtered.Values of radioactivity per milliliter were constant usingChlorellain Chu 10 medium. However, the curve could, be obtained by increasing pH and bicarbonate concentrations in the medium and on resuspending the algae in Lake Ontario (winter) filtrate. The values of cpm/ml retained from filtrates were low inCryptomonascultures and the curve was not obtained unless population density was reduced, thus increasing the relative contribution to the radioactivity on the filters.The curve was not always obtained in lake water. It was significant in 10 out of 14 experiments in Lake Ontario and in 2 out of 5 experiments in Grenadier Pond. Changes in lake water rather than in experimental techniques were probably responsible. On 2 occasions when values of cpm/ml were constant in Lake Ontario, addition of sodium bicarbonate without a pH change resulted, in a significant curve.Our experiments do not disprove the possibility of cell damage during Millipore filtration, however, it has been shown that14C labeled substances retained from solutions can account for the entire range of decreasing values as a function of volume filtered.