Mitochondria from pea leaves were purified by centrifugation on a self-generated Percoll gradient which contained a linear gradient of polyvinylpyrrolidone-25 (0-10%, w/v). The chlorophyll content of the purified mitochondria was less than 1 �g per mg protein. All substrates were rapidly oxidized by these mitochondria, the rate of glycine oxidation being between 200 and 300 nmol O2 min-1 mg-1 protein, depending on the age of the leaves used. These rates did not vary significantly over a period of 20 h, provided NAD+ was supplied exogenously, when the mitochondria were stored on ice. Respiratory control, ADP/O ratios and outer membrane integrity (always more than 95%) were also maintained during storage. The phospholipid composition of the membranes from the leaf mitochondria was virtually identical to that of mitochondria from non-photosynthetic tissues although their lipid to protein ratio was slightly lower. The polypeptide pattern of the membranes from green leaf mitochondria and those from etiolated leaves and hypocotyls were also similar, but marked differences were observed between the matrix proteins from the different tissues. In particular, intensely stained bands at 94, 51,41 and 15.5 kDa which were present in the matrix of green leaf mitochondria were missing or present in much smaller quantities in the non-photosynthetic tissues. This difference was correlated with the ability of the mitochondria to oxidize glycine, suggesting that the four polypeptides may be associated with the glycine decarboxylase complex.