Direct determination has been made of cyclobutyl pyrimidine dimer induction and excision repair in an episomal SV40 DNA population in vivo. Maintaining SV40-transformed human (GM637) cells in confluent culture results in amplification of a mutant SV40 episome to high copy number. T4 endonuclease V was used to quantify the induction and repair of cyclobutane dimers in the SV40 episome and genomic DNA of the same cells. Differences in both parameters were observed; cyclobutane dimers were induced at 1.5-2-fold greater frequency in episomal DNA and excised at a reduced rate compared to genomic DNA in the host cells.