The interaction of human serum albumin and of alpha-fetoprotein to poly(ethylene glycol)-bound fatty acids is compared using the method of affinity partitioning in an aqueous two-phase system composed of dextran, poly(ethylene glycol) and esters of different fatty acids with poly(ethylene glycol). Albumin was found to be bound to the fatty acid derivatives with higher affinity than alpha-fetoprotein. For both proteins binding occurred in dependence on the chain length of the fatty acids with a maximum for the C14-derivative.