Abstract: Heterosis has been used with success in the production of maize and sorghum. In the search for combinations of parent lines giving marked beterosis, method for rapid screening without resorting to actual crossing is needed. McDaniel and Sarkissian claimed in 1966 that the 1:1 mixture of mitochondrial preparations from seedlings of parent lines gave higher oxidative phosphorylation rates compared with that of either parent line measured singly (on protein basis). Workers from many laboratories had repeated these experiments, but not all of them were able to confirm the claim. We have made similar experiments using 32P-ATP formation method on 6 well established combinations of maize known to show marked heterosis in yield. Only 2 out of 6 combinations was mitochondrial complementation observed. Since our prepatation's of mitochondria had a ADP/O ratio of 2, not far from that obtained by Mcdaniel and Sarkissian, they were of similar quality, though the loss of activity on standing occurred earlier with our preparations. As mitochondrial complementation was not consistently observed in heterotie combinations, the validity of its use in screening is to be regarded with reservation, at least in maize.