One of the major proteins secreted by Sertoli cells (Band 3) has a molecular weight on sodium dodecyl sulfate-polyacrylamide gels which corresponds to rat serum transferrin. The preparation of Sertoli cell-secreted proteins contains an iron-binding protein with electrophoretic properties on native polyacrylamide gels similar to serum transferrin. Antibodies against Sertoli cell-secreted proteins contain a component which will immunoprecipitate serum transferrin. Antibodies to serum transferrin cross-react with and immunoprecipitate a major component (Band 3) of [35S]-methionine-labeled Sertoli cell-secreted proteins. We propose that testicular transferrin is a major secretory product of Sertoli cells in culture. In addition to transferrin, antiserum to rat serum proteins will immunoprecipitate one other polypeptide from preparations of Sertoli cell-secreted proteins.