Certain sperm components labeled with fluorescein isothiocyanate or its radioactive derivative, 125I-diiodofluorescein isothiocyanate (125IFC), are transferred at fertilization to the egg, where they persist throughout early cleavage stages at a localized site in the embryo cytoplasm (Gabel, C. A., E. M. Eddy, and B. M. Shapiro, 1979, Cell, 18:207-215; Gundersen, G. G., C. A. Gabel, and B. M. Shapiro, 1982, Dev. Biol., 93:59-72). By using image intensification we have extended these observations in the sea urchin to the pluteus larval stage, in which greater than 60% of the embryos have localized fluorescent sperm components. Because of the unusual persistence of the sperm components in the embryo, a characterization of the nature of the labeled species in sea urchin sperm was undertaken. Approximately 10% of the 125IFC was in sperm polypeptides of Mr greater than 15,000. These proteins were on the sperm surface as shown by their sensitivity to externally added proteases. The remainder of the 125IFC in sperm was in several low-molecular-weight species, none of which was 125IFC-derivatized phospholipid. To determine if any labeled sperm polypeptides remained intact in the embryo after fertilization, 125IFC-labeled sperm proteins were recovered from one-cell and late gastrula stage embryos by using an anti-IFC immunoadsorbent. Most of the labeled sperm proteins were degraded shortly after fertilization; however, distinct sets of labeled polypeptides were recovered from both one-cell and gastrula stage embryos. Six of the labeled polypeptides recovered from both embryonic stages had identical SDS gel mobilities as labeled sperm polypeptides. Other polypeptides in the embryos appeared to arise from limited proteolysis of sperm proteins. Thus, in this physiological cell fusion system, individual sperm proteins are transferred to the egg at fertilization, and some persist intact or after specific, limited degradation long after gamete fusion, until at least the late gastrula stage.