A glycoprotein unique to the cytoplasm of the unsporulated oocyst of Eimeria tenella has been purified and partially characterized. The protein has a molecular weight of 30,000, of which approximately 40% is carbohydrate. The carbohydrate portion of the molecule consists of glucose, galactose, mannose, xylose, glucosamine, and galactosamine, with no detectable sialic acid. The protein portion contains approximately 141 residues, being rich in hydrophilic amino acids with very few aromatic amino acids and no cystine. The protein comprises 14% of the total soluble protein of the unsporulated oocyst but has not been identified in the cytoplasm of any other developmental stage of the organism. Using polyacrylamide gel electrophoresis and a radioimmunoassay specific for the protein, it has been shown to disappear from the cytoplasm between the 15th and 20th hour of the 20-hour sporulation process. Subsequent immunofluorescence experiments have shown a reactive material as a component of the sporozoite membrane. These results indicate that the glycoprotein is a structural protein of the sporozoite membrane, apparently synthesized by the unsporulated oocyst and incorporated into the sporozoite membrane as one of the last steps involved in the sporulation process.