Investigations concerning the co-antibodies formed in the course of human leptospirosis and the extractive antigens of leptospiras were reviewed. As a whole, the results concerning leptospira antigens point, next to antigens of serotype, to genus-specific and intertype antigens deeply seated in the cells. These cannot be identified by the MAL test, and therefore are not related to the co-agglutinins in human leptospirosis. The associations of co-antibodies in human leptospirotic sera were analyzed, using the MAL and CF tests performed with 20 pathogenic and water serotypes of leptospira. The results showed that in the course of human leptospirosis high titer co-antibodies are formed, both agglutinating and complement fixing, variably associated for one or more serotype antigens not belonging to the infecting strain. The various biological interpretations of the phenomenon were discussed and the hypothesis of antigenic changes of leptospira strains in vivo as well as in vitro was discussed, as a determining factor in the formation co-antibodies in leptospirosis.