A variety of immune parameters are modified during and after a space flight. The effects of space flights on cellular immunity are well documented, however little is known about the effects of these flights on humoral immunity. During the Genesis space experiment, two adult Pleurodeles waltl (urodele amphibian) stayed 5 months on board Mir and were subjected to oral immunization. Animals were sacrificed ten days after their return on earth. IgM and IgY heavy chains transcripts in their spleens were quantified by northern blotting. The use of the different VH families in IgM heavy chain transcripts was also analyzed. Results were compared with those obtained with ground control animals, and animals reared in classical conditions in our animal facilities. We observed that ten days after the return on earth, the level of IgM heavy chain transcription is normal, but the level of IgY heavy chain transcription is at least three times higher than in control animals. We have also observed that the use of the different VH families in IgM heavy chain transcripts is modified by the flight. These data suggest that the space flight has affected the antibody response against the antigens contained in the food.