The sea urchin egg yolk granule is a storage compartment for HCL-32, an extracellular matrix protein. We have utilized protein gel blot analysis and immunogold labelling to define the intracellular storage compartment for HCL-32, a 32-kDa protein component of the sea urchin embryonic extracellular matrices, the hyaline layer and basal lamina. Anti-HCL-32 antiserum specifically labelled yolk granules in unfertilized eggs. Cortical granules, mitochondria, sparse granules, and lipid vacuoles were not labelled. Label continued to be detected in the yolk granules through to the blastula stage of development. However, by the gastrula stage no labelling was detected in the yolk granules. In protein gel blot analysis HCL-32 was detected in yolk granules prepared from unfertilized eggs. These results clearly define the yolk granule as a storage compartment for HCL-32, an extracellular matrix protein.