Disclosed herein are methods for making large amounts of highly pure colonization factors. The methods of the present invention differ from prior art methods in that host cells which express the colonization factor of interest are cultured in media comprising more than about 50 .mu.g/l of an antibiotic, the media is centrifuged and then filtered with a 0.2 .mu.m filter tangential flow cartridge and a 300,000 MW cut-off filter, and a divalent cation is added. As disclosed herein the colonization factors made by the method of the present invention may be used in pharmaceutical compositions and methods for treating or preventing enterotoxigenic Escherichia coli infections.