Multilamellar liposomes composed of eight different mixtures of distearoylphosphatidylcholine (DSPC) and sphingomyelin (SM) were prepared containing [99mTc]pertechnetate. The in vitro permeability of the liposome preparations in buffer and serum were measured by both dialysis and direct exposure. Liposomes composed of 25-33% SM were least permeable exhibiting leakage half-times of approximately 70 h in buffer and 52 h in serum. Pertechnetate-containing liposomes of three different lipid compositions were injected into mice and the biodistribution of 99mTc activity was determined. All preparations were taken up primarily by the lungs and liver. The half-time for clearance of 99mTc activity ranged from 24.8 to 30.6 h for the liver and 25.3 to 33.0 h for the lungs.