The phospholipid/protein ratios of rat liver peroxisomes, mitochondria and microsomes were determined and found to be 257 +/- 26, 232 +/- 20 and 575 +/- 20 nmol.mg-1, respectively. After correction for the loss of soluble protein, a peroxisomal ratio of 153 nmol.mg-1 was calculated. Organelle fractions were treated with sodium carbonate, whereafter membrane fragments containing integral membrane proteins were pelleted. For the membrane fractions of peroxisomes, mitochondria and microsomes phospholipid/protein ratios of 1054 +/- 103, 1180 +/- 90 and 1050 +/- 50 nmol.mg-1 were found, whereas 26 +/- 2, 20 +/- 2 and 49 +/- 2% of the organelle protein was recovered in these membrane fractions, respectively. The phospholipid composition of the different organelle fractions were determined, but no large differences were obtained, except for cardiolipin that was found only in the mitochondrial fraction. After sodium carbonate treatment virtually all enzymatic activity of the enzymes tested was lost. Therefore Triton X-114 phase separation was used to obtain the peroxisomal membrane components. In this fraction 42.9 +/- 3.5% of the protein and 90.2 +/- 3.7% of the phospholipid was found. Enzymatic activity of two integral membrane proteins was recovered for over 90% in the membrane fraction, whereas activity of two matrix proteins was mainly found in the soluble fraction. Urate oxidase, the peroxisomal core protein, behaved differently and was recovered mainly with the membrane components. Recoveries of enzymatic activities after the Triton X-114 phase separation varied from 45 to 116%, and together with the good separation that was obtained between soluble proteins and integral membrane proteins this method provides a useful alternative for the isolation of membrane components.