Most of the water-soluble lipoproteins in sea urchin eggs (Hemicentrotus pulcherrimus) were localized within yolk granules. Under hypotonic conditions, yolk granules released lipoproteins and a 24S protein species as high molecular weight components; the lipoproteins constituted about 40% of the total materials released. Three yolk lipoproteins (YLP-1, 2, and 3, in order of quantity) were isolated by ultracentrifugation and gel filtration. The hydrated densities of YLP-1, 2, and 3 were 1.027, 1.062, and 1.009 g/cm2 respectively. YLP-1, 2, and 3 contained glyceride as a major lipid in quantities of 3.1, 1.8, and 4.3 times the amount of each protein, respectively. These lipoproteins contained large amounts of carbohydrate. Sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed four major periodic acid-Schiff (PAS) positive polypeptide bands common to the three lipoproteins. All the constituent polypeptides of the 24S protein were also PAS positive. Electron microscopy of negatively stained YLP-1, 2, and 3 revealed the average diameters to be 36, 29, and 48nm, respectively. The 24S protein appeared to be cylindrical in shape with average exterior dimensions of 10–20 nm. Thin-section micrographs showed that yolk granules are packed with particles around 30 nm in diameter, suggesting that these particles are not the 24S proteins but the lipoprotein particles.