The most immunogenic of various acylated human serum albumin (HSA) preparations also showed the greatest anti-complementary activity. This is due in part to their ability to activate the alternative pathway and consume complement components. Such molecules also adsorbed radiolabelled complement components readily and were themselves rapidly removed from the circulation after their intravenous injection. Highly lipidated C14 HSA (C14, fatty acid side-chains with 14 carbon atoms) was not itself mitogenic: unlike HSA it localized heavily to the red pulp as well as to cells of dendritic form in the splenic white pulp. The increased immunogenicity is found in lipidated HSA species which show the following features: hydrophobicity, anti-complementary activity, ability to activate complement, heavy localization in the red pulp and an unusual tendency to localize to dendritic cells of the while pulp.