Fatty acid analysis of purified bovine alpha-foetoprotein showed it to contain 2.7 mol of fatty acid/mol of alpha-foetoprotein. Purified alpha-foetoprotein focused at isoelectric point 4.8. Removal of bound ligands from alpha-foetoprotein by charcoal treatment changed its isoelectric point to 5.2. This change could be reversed by addition of exogenous fatty acids to the defatted alpha-foetoprotein. Albumin isolated from the same foetal calf serum source as alpha-foetoprotein contained 1.4 mol of fatty acid/mol of protein. alpha-Foetoprotein and albumin contained comparable amounts of fatty acids with 14 to 18 carbon atoms, but alpha-foetoprotein contained 16 times as much of the long-chain polyunsaturated fatty acids as albumin. alpha-Foetoprotein was found to have slightly higher affinity for palmitate and linoleate and severalfold higher affinity for arachidonate than albumin. These findings suggest that alpha-foetoprotein may play a role in the foetal metabolism of the long-chain polyunsaturated fatty acids.