Hypermutation of immunoglobulin genes is a key process in antibody diversification. Little is known about the mechanism, but the availability of rapid facile assays for monitoring immunoglobulin hypermutation would greatly aid the development of culture systems for hypermutating B cells as well as the screening for individuals deficient in the process. Here we describe two such assays. The first exploits the non-randomness of hypermutation. The existence of a mutational hotspot in the Ser31 codon of a transgenic immunoglobulin V gene allowed us to use PCR to detect transgene hypermutation and identify cell populations in which this mutation had occurred. For animals that do not carry immunoglobulin transgenes, we exploited the fact that hypermutation extends into the region flanking the 3′-side of the rearranged J segments. We show that PCR amplification of the 3′-flank of VDJ _H rearrangements that involve members of the abundantly-used V _H J558 family provides a large database of mutations where the germline counterpart is unequivocally known. This assay was particularly useful for analysing endogenous immunoglobulin gene hypermutation in several mouse strains. As a rapid assay for monitoring mutation in the J _H flanking region, we show that one can exploit the fact that, following denaturation/renaturation, the PCR amplified J _H flanking region DNA from germinal centre B cells yields mismatched heteroduplexes which can be quantified in a filter binding assay using the bacterial mismatch repair protein MutS [Wagner et al . (1995) Nucleic Acids Res . 23, 3944–3948]. Such assays enabled us, by example, to show that antibody hypermutation proceeds in the absence of the p53 tumour suppressor gene product.