Mitochondrial DNA was quantitated in total DNA of various normal and mutant strains of human diploid fibroblasts (finite replicative lifespan) and permanent cell lines, using Southern-transfer hybridization to 32P-labeled pure mtDNA probe and saturation hybridization to 3H-labeled cRNA copied from mtDNA. In six normal fibroblast strains, mtDNA copy number increased during serial passage roughly in proportion to cell volume or protein content, whereas normalized mtDNA content per pg of protein depended upon in vivo donor age but not passage level (“in vitro” age). Copy numbers for mtDNA varied much more widely in individual fibroblast clones than in mass cultures, but were not well correlated with longevity or growth rate. Five mutant fibroblast strains associated with reduced replicative lifespan, and four permanent cell lines, were also examined; in each group, mtDNA values were observed both lower and higher than any obtained for normal fibroblasts. No evidence was found of petite-type deletions from human mtDNA, either at late passage or in individual clones of fibroblasts. Methylation of mtDNA genomes was strikingly non-random and apparently decreased with culture age.