Sera collected during the 1985 oral cholera vaccine trial in Matlab, Bangladesh, which demonstrated efficacy of a whole cell combined with cholera B subunit vaccine (WC/BS) and a whole cell only vaccine (WC), were analyzed for antitoxin and vibriocidal antibodies. Before vaccines were given, antitoxin titers were highest in children, especially those with O blood group, whereas vibriocidal titers rose throughout life. Two weeks after three doses of vaccine, geometric mean antitoxin titers were 2.5-4.5 times higher in vaccinees who received the WC/BS vaccine; the vibriocidal titers were 1.3-2.1 times higher in vaccinees who received either vaccine. The titer elevations were relatively brief and were barely detectable 7 months after the third dose even though significant levels of protection persisted greater than or equal to 3 years. Thus, the oral vaccines induced a serum response in this large field trial that was similar to that seen in earlier pilot studies, but the duration of the serum responses was much shorter than the duration of the protection.