Photoreactivation and pedigree analysis have been combined to show that unexcised pyrimidine dimers in the DNA of rad1-1 yeast can initiate mutagenesis after passing through several DNA replications. Monomerisation of dimers immediately before the second replication to follow UV has no effect on mutants appearing after the first post-UV cell division but reduces second-generation mutants to one third of their frequency in the dark and has a similar through slightly less marked effect on mutants appearing in the third or subsequent generations. The bearing of these findings on the mechanism of UV mutagenesis is dicussed.