Novel synthetic lipid derivatives of poly(ethylene glycol) (PEG) have been synthesized and tested for their ability to decrease uptake of liposomes into the mononuclear phagocyte system (MPS, reticuloendothelial system) in mice and to prolong circulation half-lives of liposomes. A carbamate derivative of PEG-1900 with distearoylphosphatidylethanolamine (PEG-DSPE) had the greatest ability to decrease MPS uptake of liposomes, at optimum concentrations of 5-7 mol% in liposomes composed of sphingomyelin/egg phosphatidylcholine/cholesterol (SM/PC/Chol, 1:1:1, molar ratio). Results obtained with this compound were equivalent to results previously obtained with 10 mol% monosialoganglioside GM1 in liposomes of similar compositions (Allen, T.M. and Chonn, A. (1987) FEBS Lett. 223, 42-46). Non-derivatized methyl PEG or PEG-stearic acid (PEG-SA) were incapable of decreasing MPS uptake of liposomes. PEG-Chol and PEG-dipalmitoylglycerol (PEG-DPG) were intermediate in their effects on MPS uptake. Altering liposome size for liposomes containing PEG-DSPE resulted in only minor changes in blood levels of liposomes. Half-lives of 0.1 microns liposomes of SM/PC/Chol/PEG-DSPE (1:1:1:0.2, molar ratio) in circulation was in excess of 20 h following either i.v. or i.p. injection. Liver plus spleen liposome levels for these liposomes was below 15% of injected label at 48 h following i.v. liposome injection and below 10% following i.p. injection. The major site of liposome uptake was in carcass tissues, with over 50% of label remaining in vivo at 48 h post-injections, either i.v. or i.p., in the carcass.