An increasing problem in cell and molecular biology is the preparation of antibodies specific to proteins that are present in minute quantities within cells or tissues. With the advent of recombinant DNA technology, it is now often possible to deduce the primary amino acid sequence of a polypeptide without its purification. Two strategies then exist to raise appropriate antibodies. Either the gene can be expressed in a heterologous species, usually bacteria, and the resultant protein used as an immunogen, or alternatively, small synthetic peptides can be made that contain amino acid sequences inferred from that of the gene. Such antipeptide antibodies crossreact with the intact native protein with surprisingly high frequency and have the additional advantage that the epitope recognized by the antibody is already well defined (1). In this way, antibodies can be raised against novel gene products that are specifically directed against sites of interest, for example, unique regions, highly conserved regions, active sites, extracellular or intracellular domains. Moreover, the ready availability of the pure peptide immunogen against which the antibody was raised means that sera can be rapidly and easily screened, e.g., using an enzyme-linked immunosorbent assay (ELISA) for antipeptide activity. Free peptide can also be used to block antibody binding and so demonstrate immunological specificity, and it may be coupled to a solid support (e.g., agarose) to generate an affinity matrix for antibody purification.