showed MLV particles with a weightaverage diameter of 152 nm as compared to the 232 nm found in HN buffer. This represents a water loss of 67 percent in the MLV and an increase in particle density to 1.0472 g/cm3, as compared to the 1.0154 g/cm3 assumed for the original particles.The particle size distribution data for the SUV particles are believed to be reasonably accurate even though no correction for the osmotic effect of the sucrose-buffer mixture was used in the measurement. The osmotic activity for the much smaller SUV particles is expected to be considerably less than'that for MLV as a result of molecular constraints caused by the'minimal radius of the SUV particles.In the SFFF analytical mode, it is relatively easy to collect submilligram quantities of isolate with very high resolution. If the resolution is compromised somewhat, milligram to gram quantities can be isolated by having the instrument function as a selective filter (3), retaining large liposomes while allowing smaller ones to be eluted in the mobile phase. In conjunction with extrusion techniques (7, 9), SFFF should make possible the rapid preparation of appreciable amounts of liposomes with a polydispersity close to unity. It thus appears that SFFF is a rapid, precise, and gentle method for the analysis of. the particle size distributions of noninteracting biological colloids, as we have illustrated with liposomes. We believe that SFFF will be an equally important technique in the fractionation and size analysis of other biop4 as nucleic acids and cellul such as ribosomes, mitoc] nuclei.

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