The present invention relates to detoxified and immunologically active proteins (“mutant LTs”) having mutated amino acid sequences of heat-labile enterotoxin of E. coli, DNA sequences encoding the mutant LTs, recombinant expression vectors comprising the DNAs, recombinant microorganisms transformed with the recombinant expression vectors, process for preparing the mutant LTs and pharmaceutical application of the said protein as immunogenic antigens for vaccination and as adjuvants for anti-body production. In contrast to wild-type LT, the mutant LTs did not induce any toxic activities. The mutant LTs elicited high and comparable levels of anti-LT antibodies when delivered either intragastrically or intranasally, inducing systemic and local responses in serum and fecal extracts. Thus, they might be useful for the development of a novel diarrheal vaccine in humans and animals. In addition, the antibody production ability using mutant LTs as an adjuvant may be effective for prevention and treatment of various diseases.