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Supernatant protein factor (SPF), a protein that stimulates squalene epoxidation, mediates the transfer of squalene between two separable microsomal populations (Kojima, Y., E. J. Friedlander, and K. Bloch, 1981. J. Biol Chem. 256: 7235-7239). We now show that SPF also promotes the transfer of squalene associated with mitochondria or with plasma membranes to total microsomes or rough or smooth microsomal subfractions. Both rough and smooth microsomes have squalene epoxidase activity that is stimulated by SPF.