Publisher Summary This chapter presents a study where monoclonal antibodies were raised against human alphafoetoprotein. One clone, AFP 21.2, secreted an IgG 1 immunoglobulin of sufficient avidity to allow discrimination of serum levels of AFP over the whole range from low to high levels in a single four-hour two-site immunoradiometric assay (IRMA). Another clone, AFP 21.1, produced immunoglobulin of low avidity that has proved ideal as a solid phase reagent for a single-step purification of the antigen under non-denaturing conditions of elution with 2M MgCl 2 . Similar gentle immunopurification systems are required for a number of products to be recovered from bacterial culture systems following DNA cloning. h-AFP is also produced by a number of tumors. Monoclonal antibodies for immunohistochemistry should ideally be directed to epitopes not destroyed by classical fixation methods. Neither AFP 21.1 nor 21.2 are apparently suitable for this.