The yellow crescent or myoplasm is a localized cytoplasmic region in eggs of the ascidian Styela that is partitioned to the larval tail muscle and mesenchyme cells during embryonic development. To determine whether the myoplasm contains a specific subset of maternal macromolecules, its abundant proteins and mRNAs were identified and compared to those present in the remainder of the egg. This was accomplished by exploiting a newly developed method for the mass fractionation of yellow crescents which is based on the presence of a unique cytoskeletal domain in the yellow crescent region. The fractionation yields a yellow crescent fraction (YCF) representing the myoplasm and a supernatant fraction representing the nonmyoplasmic regions. The YCF comprises structures which contain the characteristic myoplasmic organelles and about 10% of the protein, 8% of the RNA, and 1-3% of the poly(A) of whole eggs. Two-dimensional gel electrophoresis indicated that the YCF contains 15 polypeptides that are undetectable in the supernatant fraction and 43 polypeptides that are significantly depleted in the latter fraction. The proteins restricted to the YCF are both of cytoskeletal and noncytoskeletal origin. In vitro translation of RNA in a message-dependent lysate and analysis of [35S]methionine-labelled polypeptide products by two-dimensional gel electrophoresis did not reveal qualitative differences between the YCF and the supernatant fraction. Furthermore, the mRNAs coding for two polypeptides that were localized in the myoplasm were not restricted to the YCF. The results suggest that qualitative differences in proteins but not in prevalent mRNAs exist between the yellow crescent and the other cytoplasmic regions of Styela eggs.