Summary. The spermicidal activities against human spermatozoa of a range of non-ionic, anionic and cationic surface-active agents have been determined with the method of Baker, Ransom & Tynen (1937). Although it has been concluded that surface-tension-lowering properties do not directly influence spermicidal power, the surfacetension/concentration curves of a number of the compounds were studied. Solubilities in water, hexane, and ethyl oleate were also determined in an attempt to find out whether there was any relationship between spermicidal action and solubilities in these solvents. The results obtained on the compounds studied showed broadly that appreciable oil solubility and good surface-tension-lowering properties were desirable for spermicidal activity in non-ionic and anionic surface-active agents. The cationic compounds studied were active in spite of low fat solubility. The lack of precision of the spermicidal results made definite activity/ solubility or activity/surface-tension relationships difficult to formulate.