An asymmetric membrane comprising a tubular polymer film in combination
with a tubular braid on which the film is supported, requires the braid
be macroporous and flexible, yet sufficiently strong to withstand
continuous flexing, stretching and abrasion during use for
microfiltration (MF) or ultrafiltration (UF). The specifications for a
braid of a long-lived membrane are provided. A membrane is formed by
supporting a polymer film in which particles of calcined .alpha.-alumina
are dispersed, on the defined tubular braid.