It has been shown that mitochondria from human placenta incubated in the presence of 2-oxoglutarate or its precursors utilize acetoacetate at the rate about I nmol/min/mg protein. Utilization of acetoacetate is completely inhibited by arsenite. Mitochondria from human placental tissue show activity of 3-oxoacid-CoA transferase and acetoacetyl-CoA thiolase. It is proposed that acetoacetate utilization by placental mitochondria proceeds via the conversion to acetoacetyl-CoA catalysed by 3-oxoacid-CoA transferase, and then to acetyl-CoA, catalysed by acetoacetyl-CoA thiolase.