Two mutant genes coding for two different variants of transthyretin were identified in two independent kindreds with familial amyloidotic polyneuropathy. A single base change from A to G was identified in exon 2 of transthyretin gene in two brothers from the first kindred. This base change led to replacement of glutamate by glycine at position 42 of 127-residue molecule. In a patient from the second kindred, T to G transversion in exon 3 of transthyretin gene led to replacement of Ser by Arg at position 50. The two mutants were discovered by randomly sequencing recombinant clones containing the entire length of each one of the four exons selectively amplified by polymerase chain reaction. The base change produced a new restriction site for Hae III and Cfr 13 I in the exon 2 and for Mva I in the exon 3, respectively. Restriction fragment length polymorphisms and allele-specific oligonucleotide hybridizations confirmed the base changes. The accurate detection of the new mutant genes is hereafter possible by these procedures.