Abstract Yeast mitochondrial (mtDNA) 3H-labelled was isolated from exponential phase cells after ultraviolet light irradiation. Both the size and amount of mtDNA were found to be reduced during a 40-h liquid-holding (LH) period in non-growth medium following irradiation as compared to the mtDNA recovered from nonirradiated cells under similar conditions. After the LH period, previously irradiated cells were suspended in growth medium containing [ 14C]adenine. Double labelled mtDNA ( 3H and 14C) was isolated from cells samples removed during new growth. A recovery in the amount and size of mtDNA was observed in irradiated cells during new growth. These biochemical studies agree with the observed loss and recovery of mtDNA genetic markers in UV-irradiated exponential phase yeast after a period of LH and new growth resp.