A glycoprotein was isolated from goat erythrocyte membranes by extraction with hot 75% ethanol. The glycoprotein was purified by ethanol precipitation, phosphocellulose chromatography gel filtration, ethyl:ether and chloroform:methanol extraction. In aqueous phosphate-buffered saline, pH 7, the glycoprotein was in an aggregated state with a sedimentation coefficient (S(obs)) to 1.5. Electrophoresis of the glycoprotein on polyacrylamide gels containing phosphate-buffered 0.1% SDS gave a single band, staining with both periodic acid Schiff (PAS) and Coomassie Blue (CB). The apparent m.w., calculated from retardation coefficient, was 25,000. Electrophoresis of the glycoprotein on 1% SDS gels buffered with Tris-acetate (pH 7.4) showed a major band of similar (23,500) apparent m.w. plus four other PAS- and CB-staining bands of lower mobility. With 131I-labeled glycoprotein, recovery of bands from gels, sialic acid analysis, heterophile antigen activity, and re-electrophoresis, it was shown that these additional bands were aggregated forms of a single or closely related glycoprotein species. The purified glycoprotein contained 50% carbohydrate with molar ratios of sialic acid:galactose:mannose:galactosamine:glucosamine of 3.1:2.1:0.1:1.6:1. The glycoprotein was highly reactive with the Paul-Bunnell heterophile antibody in the sera of patients with infectious mononucleosis, with Limulus polyphemus lectin and weakly ractive with wheat germ agglutinin. These reactivities were destroyed by neuraminidase treatment or by alkaline sodium borohydride. The native glycoprotein did not react with lectins from Canavalia enisformis, Phaseolus vulgaris, Ricinus communis, or Vicia graminea although it was reactive with the latter two after neuraminidase treatment.