Albumin is a multifunctional transport protein that binds a wide variety of endogenous substances and drugs. Insulins with affinity for albumin were engineered by acylation of the epsilon-amino group of LysB29 with saturated fatty acids containing 10-16 carbon atoms. The association constants for binding of the fatty acid acylated insulins to human albumin are in the order of 10(4)-10(5) M-1. The binding apparently involves both non-polar and ionic interactions with the protein. The acylated insulins bind at the long-chain fatty acid binding sites, but the binding affinity is lower than that of the free fatty acids and depends to a relatively small degree on the number of carbon atoms in the fatty acid chain. Differences in affinity of the acylated insulins for albumin are reflected in the relative timing of the blood-glucose-lowering effect after subcutaneous injection into rabbits. The acylated insulins provide a breakthrough in the search for soluble, prolonged-action insulin preparations for basal delivery of the hormone to the diabetic patient. We conclude that the biochemical concept of albumin binding can be applied to protract the effect of insulin, and suggest that derivatization with albumin-binding ligands could be generally applicable to prolong the action profile of peptide drugs.