Serum sphingomyelins were analyzed by argentation chromatography of the corresponding ceramide diacetates. Six subfractions were obtained. Three of them contained 4-sphingenines in combination with saturated, trans-, or cis-monoenoic fatty acids; the remaining three contained sphingadienine, also in combination with saturated, trans-, or cis-monoenoic fatty acids. Palmitic acid was the principal fatty acid combined with 4-sphingenines, while nervonic acid was the principal fatty acid combined with sphingadienine. About 4% of the total fatty acids of sphingomyelin were trans-monoenoic. They were comprised of many positional isomers of straight-chain C(22-24) compounds. The cis-monoenoic acids made up 33% of the total acids and consisted of almost pure nervonic acid. The rest of the acids were saturated. The 4-sphingenines contained small amounts of iso-C(18) and anteiso-C(19) compounds in addition to the straight-chain C(16-18) bases.