The regional differentiation of the sperm surface has been studied with the aid of a novel covalent labeling technique that permits concurrent cytological, biochemical, and immunological analyses. For these studies isothiocyanate derivatives of fluorescein (FITC) and diiodofluorescein (IFC) were employed: the latter can be prepared with radioiodine to high specific activity (125IFC) and is an impermeant reagent for the erythrocyte surface. Sperm of sea urchin (Strongylocentrotus purpuratus), medaka )Oryzias latipes), and golden hamster bind the fluorescent chromophores with a nonuniform distribution, most of the fluorescence being associated with the midpiece. The radioactive derivative 125IFC permits an analysis of the proteins that are responsible for most of the binding. Additionally, 125 IFC-labeled sperm are capable of fertilizing eggs, as assessed by autoradiography. That IFC labels the surface of the sperm was inferred from the following: (a) the labeling of the surfaces of other cells by fluorescein isothiocyanate and its derivatives; (b) the agglutination of labeled sperm by antibodies directed against IFC; (c) the use of peroxidase-dependent immunocytochemical reaction using anti-IFC antibodies, with analysis by electron microscopy; and (d) extraction of labeled sea urchin sperm with Triton X-100 under conditions that preferentially solubilize the plasma membrane. The antiserum directed against IFC was used to isolate the labeled surface components from Triton X-100 extracts of whole sperm, by immunoprecipitation, with Staphylococcus-A protein serving as a coprecipitant. The results support previous data showing that the sperm surface is a heterogeneous mosaic of restricted domains, one notable zone being the midpiece, where common molecular properties may be shared by sperm with distinctly different morphologies. In addition, IFC-mediated covalent alteration of specific cell surface proteins may be used to label, to identify, and, with the use of anti-IFC antibodies, to isolate such proteins from other cellular constituents.