Natural proteins such as casein and enzymes cannot be processed into fibrous forms. With the addition of another polymer such as poly( ethylene oxide) (PEO) or poly( vinyl alcohol) (PVA), casein, a natural protein, was electrospun into ultra-thin fibrous membranes. Fibers in these membranes had diameters between 100 and 500 nm. These fibrous membranes were rendered insoluble by chemical crosslinking with 4,4'-methylenebis(phenyl diisocyanate) (MDI) in THF. The electrospinning method has also been utilized for enzyme immobilization on solid support. The lipase encapsulated in the ultra-fine fibrous membranes exhibits higher catalytic activity towards hydrolyzing olive oil than that in the cast films from the same solution. (C) 2003 Kluwer Academic Publishers.