There is considerable evidence that mouse fertilization requires the binding of sperm to two of the three glycoproteins that form the zona pellucida (ZP), ZP3 and ZP2. Despite the biologic importance of this binding, no one has demonstrated that sperm express separate, saturable, and specific binding sites for ZP3 and for ZP2. Such a demonstration is a prerequisite for defining the distribution, numbers, affinities, and regulation of function of ZP3 and ZP2 binding sites on sperm. The experiments reported herein used fluorochrome-labeled ZP3 and ZP2 and quantitative image analysis to characterize the saturable binding of ZP3 and ZP2 to distinct sites on living, capacitated, acrosome-intact mouse sperm. Approximately 20% of the ZP3 binding sites were found over the acrosomal cap, and the remaining sites were located over the postacrosomal region of the head. In contrast, ZP2 binding sites were detected only over the postacrosomal region. Saturation analysis estimated numbers and affinities of the binding sites for ZP3 (B(max) approximately 185 000 sites per sperm; K(d) approximately 67 nM) and ZP2 (B(max) approximately 500 000 sites per sperm; K(d) approximately 200 nM). Use of unlabeled ZP3, ZP2, and ZP1 as competitive inhibitors of the binding of fluorochrome-labeled ZP3 and ZP2 demonstrated that ZP3 and ZP2 bound specifically to their respective sites on sperm. Finally, we demonstrate that extracellular calcium as well as capacitation and maturation of sperm are required for these sites to bind their respective ligands.

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Paleomagnetic studies of folded and flat-lying volcanic rocks in southwestern Montana indicate that most of the natural remanent magnetization (NRM) is primary and that secondary magnetization, where found, has been produced mainly by lightning. NRM directions of most rock units are grouped in either a normal or reversed sense after partial demagnetization, although directions of a few units of probable mudflow origin are highly dispersed. No evidence of self-reversed magnetization has been found; consequently, a field polarity sequence has been tentatively established in connection with Late Cretaceous and early Tertiary orogenic events. All folded rocks investigated belong to the Upper Cretaceous Elkhorn Mountains Volcanics; stratigraphic correlation between widely separated, structurally dissimilar outcrops of petrologically similar rocks is supported by NRM directions and polarities. Mean NRM directions of structurally restored, age-equivalent sites do not differ significantly from one another, which suggests that fold axes are nearly horizontal; however, they differ significantly from directions of nearby fiat-lying volcanic rocks of probable Tertiary age. The virtual geomagnetic pole calculated for the Elkhorn Mountains Volcanics (69øN, 171øW) agrees generally with Late Cretaceous poles previously calculated by others for North America, Madagascar, and the U.S.S.R., but it differs significantly from Early Cretaceous poles of Japan and Australia. •[NTRODUCTION The remanent magnetization of Upper Cretaceous volcanic rocks in southwestern Montana was studied as an aid to interpreting the geologic history. Rocks suitable for paleomagnetic investigations were sampled from the foothills of Bull Mountain and other localities within a 12,500-km •' area that is centered near 46øN, 112øW (Figure 1). Paleomagnetism of materials from Bull Mountain is of particular significance for two reasons--(1) the geologic age of the rocks there has been established and (2) the structural relations in the area are well known.

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