The ability of membrane antigens on sporozoites of the intestinal pathogen, Cryptosporidium parvum, to bind host cell surface antigens was investigated. A novel membrane-associated protein of approximately 47 kDa, designated CP47, was found to possess significant binding affinity for the surface of both human and animal ileal cells. This protein was purified by a combination of anion-exchange chromatography on FPLC and immunoaffinity chromatography. Purified CP47 demonstrated competitive binding with parasite-associated membrane antigens to membranes of HCT-8 and ileal cells in a dose-dependent manner. Furthermore, the binding activity of CP47 was found to be Mn²⁺-sensitive, and was completely inhibited in the presence of 10 mM MnCl₂. These results were consistent with earlier findings demonstrating the inhibitory effect of Mn²⁺ ions on Cryptosporidium infection both in vitro and in vivo (Nesterenko et al., Biol. Trace Elem. Res. 56 (1997) 243–253). Immunoelectron microscopy using gold-conjugated antibodies revealed CP47 to be localized at the apical end of the sporozoites. A single protein with an electrophoretic mobility of 57 kDa was purified from host cell membranes using CP47-Affigel. Similarly, affinity purification of this protein was abrogated in the presence of Mn²⁺. These data suggest that a novel parasite protein, CP47, may play an important role in sporozoite/host cell attachment.