Removal of [2-14C] 2,4-D [(2,4-dichlorophenoxy)-acetic acid] from water was pronounced with Scenedesmus quadricauda (Turp.) Breb. and most effective uptake occurred at pH 4.7. Three other algae were relatively ineffective in removal of 2,4-D. Increasing Scenedesmus cell density increased 2,4-D removal, but the amount absorbed was not a linear function of cell density. Similarly, with a range of herbicide concentrations (0.01 to 1.0 ppmw) the amount removed was greatest at the highest concentration; however, a higher percentage of chemical was removed at lower concentrations. An equilibrium of 14C removal from water and 14C released to the water was reached by 4 hr. More herbicide was removed from water by Scenedesmus in darkness than in light and low temperature (2 C) reduced uptake slightly. Relatively high 2,4-D concentrations were required to reduce growth in any of the four algae and Scenedesmus was most sensitive. Scenedesmus metabolized [ring
−14C] in 24 hr to several products which were found in suspension fluid and cells. Quantitatively, 3-OH-2,4-D [3-hydroxy-2,4-dichlorophenoxy)acetic acid] was the major metabolite, comprising 26% of the radioactivity extracted from cells compared to 15% from suspension fluid. A smaller amount of 5-OH-2,4-D [(5-hydroxy-2,4-dichlorophenoxy)acetic acid] was isolated from the suspension fluid and none from the cells.