A number of bacterial polysialic acids were injected intravenously into mice. Half-lives (up to 40 h) in the blood circulation were dependent on the polysialic acid used, increased by deacylation of their phospholipid moiety, decreased with shorter chain derivatives and appeared to be dose independent. A model drug (fluorescein) covalently coupled to a polysialic acid was found to assume the half-life of its carrier. Results suggest that intact or deacylated polysialic acids and shorter chain derivatives can be used to augment the half-lives of drugs, small peptides, proteins and drug delivery systems in the blood circulation, thus prolonging their pharmacological action.