Mutation at the human minisatellites MS32, MS205 and MS31A has been investigated by characterizing mutant alleles in pedigrees and in the case of MS32 by direct analysis of mutant molecules in single sperm. Most mutations at all three loci are polar, involving the preferential gain of a few repeat units at one end of the tandem repeat array. Incoming repeats can be derived from the same allele or the homologous chromosome, through they are frequently rearranged during mutation. Lack of exchange of flanking markers suggests the involvement of complex conversion-like events in the generation of mutant alleles. At MS32, high frequency mutation processes in sperm appear to be largely germline specific and to occur at a constant rate irrespective of allele size. Together with mutational polarity, this implies that germline instability is controlled by elements outside the tandem repeat array.