We have established three murine hybridoma cell lines that secrete monoclonal antibodies directed specifically against the alpha subunit of the DNA polymerase III holoenzyme of Escherichia coli. All three antibodies have been purified and identified to be of the IgM class. competition binding assays indicate that these antibodies bind to at least two distinct but adjacent or interacting sites. An immunoblot assay has been developed that permits quantitation of alpha in crude extracts. These assays agree with previous determinations of the number of polymerase molecules present/cell and indicate that the size of alpha is the same in both the crude and pure form. Anti-alpha IgM can be used with fixed Staphylococcus aureus, cells and anti-IgM to specifically precipitate alpha.