In a controlled field trial on some 584 000 people in an endemic cholera El Tor area in the Philippines, it was demonstrated that cholera vaccines gave moderate protection of short duration. Injection of a single dose of vaccine prepared from either Vibrio cholerae or Vibrio El Tor gave over 50% protection for the first two months. The immunity conferred by the V. cholerae vaccine rapidly declined after three to four months. The V. El Tor vaccine gave protection for six months, but its effectiveness declined. An oil-adjuvant vaccine prepared from V. cholerae conferred an increasing degree of protection of long duration, but, owing to severe vaccination reactions, its use could not be recommended.