This chapter discusses the preparation and purification of a concanavalin A-trypsin conjugate. Con A-trypsin provides an experimental tool to investigate interrelationships in the modes of action of various mitogens. Insulin, proteolytic enzymes, and Con A all exert mitogenic activity with selected cell types, and the results of a variety of studies suggest possible interrelationships between their mechanisms of action in some cell types. Lectins as carriers of enzymes and drugs have several advantages over monoclonal antibodies. These include (1) the commercial availability of most lectins in greater quantities and at lower cost, (2) the fact that both the lectin and conjugates containing it are readily purified on readily available, reusable affinity columns, and (3) prolonged retention of lectins at the site of injection in tissues. The property of prolonged retention of lectins at the site of injection in tissues has potential applications for controlled release of drugs, although the toxicity and immunogenicity generally associated with lectins have limited its potential application in this area.