AbstractMitochondria prepared from the first growth of cells (fibroblasts) from skin biopsies from homozygous (but not heterozygous) achondroplastic human subjects were unable to carry out oxidative phosphorylation. However, successive crops of cells gained the ability to phosphorylate with normal P:O ratios with pyruvate‐malate and succinate as substrates. Concentrations of cytochromes a + a3were markedly and significantly lower in homogenates of homozygous achondroplastic tissue culture cells than in homogenates of normal cells. Levels of cytochromes a + a3in the heterozygous achondroplastic cells were intermediate between the levels in normal cells and the homozygous achondroplastic cells. Activities of the mitochondrial oxidative systems (NADH, succinic and cytochrome oxidases) were not significantly lower in the achondroplastic cell preparations than in normal cell preparations under standard assay conditions (saturation levels of oxygen