We report here the presence in human milk fat globules membranes of 5'-nucleotidase, Na(+)-K(+) and Mg(2+) ATPases, and phosphodiesterase which are marker enzymes for the plasma membrane. Thiamine-pyrophosphatase, lactose and lactosamine synthetase were also found, which are usually considered as Golgi apparatus marker enzymes. Lastly, glucose-6-phosphatase, NADH-cytochrome c reductase and RNase, characteristic enzymes of the endoplasmic membrane, were also present.