The following fractions can be obtained after extraction of biological samples with ODS silica. 1. 1. Arachidonic acid or other nonpolar fatty acids. Apply sample to ODS silica in acidified 30% ethanol. Wash with 30% ethanol (20 ml), water (20 ml), and petroleum ether (10 ml). Elute 20:4 with petroleum ether-chloroform (1:1, 20 ml). 2. 2. Monohydroxyeicosenoic acids. Apply sample in acidified 15–25% ethanol, depending on whether it is desired to obtain a subsequent fraction containing PGs. Wash ODS silica with the same concentration of ethanol (20 ml), water (20 ml), and petroleum ether (20 ml). Elute monohydroxyeicosenoic acids with petroleum ether-chloroform (65:35, 20 ml), chloroform (10 ml) or methyl formate (10 ml), depending on whether or not it is necessary to obtain a subsequent PG fraction. 3. 3. Prostaglandins and thromboxanes. Apply sample to ODS silica in acidified 15% aqueous ethanol. Wash with 15% ethanol (20 ml) followed by petroleum ether (20 ml) or, if it is desired to extract monohydroxyeicosenoic acids, water (20 ml), and petroleum ether-chloroform (65:35, 20 ml). If nonpolar PGs such as 13, 14-dihydro-15-oxo-PGE 2 are to be extracted, petroleum ether should be used. Elute PGs and Txs with methyl formate (10 ml). 4. 4. Monohydroxyeicosenoic acids, PGs, and Txs. Apply sample in acidified 15% aqueous ethanol. Wash ODS silica with 15% aqueous ethanol (20 ml), water (20 ml) and petroleum ether (20 ml). Elute with methyl formate (10 ml). 5. 5. Polar PG metabolites. Apply sample in acidified 0–10% aqueous ethanol (for ω-hydroxy-PGs 10% ethanol can be used). Wash ODS silica with the same concentration of ethanol (20 ml) followed by petroleum ether-chloroform (65:35). Elute PG metabolites with methyl formate (10 ml). 6. 6. PGs extracted from neutral or basic media. Apply sample to the ODS silica in 10–15% aqueous ethanol at the desired pH. Wash with the same concentration of ethanol (10 ml) (for greater speed, it should be possible to eliminate this step), followed by petroleum ether (20 ml). Elute with methyl formate (10 ml). For nonpolar products, better results might be obtained by washing the ODS silica with water prior to petroleum ether.