A new method for the covalent radioactive and spin labelling of lipids within isolated biological membranes has been described in detail and applied to studies of temperature-induced changes of microsomal and mitochondrial membranes. The method is based on the enzymatic use of radioactive substrates carrying covalently bound doxyl derivatives of stearic acid in the biosynthesis of phospholipids in isolated membranes. Radioactive-and spin-labelled lipids bound to the microsomal and mitochondrial membranes were then used as internal probes in monitoring temperature-induced changes of these membranes. Since the analysis of isolated radioactive-and spin-labelled lipids revealed the exact composition of membrane-bound labelled lipids, specific temperature-induced changes were correlated with specific lipids of examined membranes. Phosphatidylinositol of microsomal membranes and polyglycerophosphatides (phosphatidyl-glycerol and cardiolipin) of mitochondrial and inner mitochondrial membranes were found to be involved in the apparent formation of lipid clusters at around 20-30 degrees C. Cardiolipin was found to be involved in the fluidization of inner mitochondrial membranes. These findings are discussed in view of the present state of knowledge of the organization of lipids in biological membranes.