A large, external glycoprotein with antigenic properties isolated from the ciliate Pseudomicrothorax dubius was found to have a molecular weight of ∼250,000 daltons. Analysis of the extracts by isoelectric focusing in combination with immunodiffusion and gradient polyacrylamide gel electrophoresis revealed that the principal antigen was a large glycoprotein. the glycoprotein was purified partially by Sephadex ultrafiltration. and almost completely by affinity chromatography on a concanavalin A‐Sepharose column.