Two discrete supranucleosomal particles that differ in their electrophoretic migration on 1% agarose gels were isolated from unfertilized sea urchin eggs. Both particles contain the same complement of cleavage stage (CS) chromosomal proteins, which is identical to the complete set of basic proteins isolated directly from chromatin by extraction with 0.25 N HCl. DNA fragments between 210 and 1500 bp were found in both particles, and the basic unit of DNA repeat length determined by micrococcal nuclease digestion was 126 +/- 3 bp. The isolated nucleoparticles are electrophoretically stable over a wide range of DNA sizes (126-1500 bp) indicating that their structure is maintained by internal interactions among the CS chromosomal proteins, previously designated CS A through CS G. Based on these results we conclude that the CS chromosomal proteins are functionally equivalent to classical histones in their ability to direct higher ordered structures of chromatin.