Abstract: 13 hybridoma cell lines secreting monoclonal antibody against Anabaena azollae have been established by fusion of SP2/0 myeloma cells with spleen cells from BALB/C mice immunized with Anabaena azollae. The chromosome numbers of the hybridoma cell lines range from 86 to 105, respectively. The antibodies from the culture medium have flourescencc antibody(FA)titers of up to 5-80, and those from mouse ascitic fluid have titers ranging from 102 to 105.Among the 13 monoclonal antibodies(McAbs), 11 McAbs react with all Anabaena azollae from seven species of Azolla. One(McAb-Cl6)reacts only with the symbionls from Euazolla, it did not react with that from Rhizosperma i. e., it is a subgroup-specific McAb. Another one(McAb-P2), which reacts only with the symbiont from A. pinnata, is a species-specific McAb.Two McAbs(Rl5 and P2)belong to IgG1 and IgG2a subclasses, respectively. The McAbs(Cl6, Me3,P1,P3,P6, R2 and R6)belong to IgG3 subclnss, and the remaining four McAbs(P5, R1, R8 and R9)belong to IgM subclass. All 13 McAbs did not react with free-living nitrogen fixing algae, Anabaena azotica and Tolypothrix tennis.The direct FA for Anabaena azollae was established with fluorescein isothiocyanate(FITC)labelled McAb-Cl6. The titer of McAb-Cl6 conjugated reached 1280, and the specific reaction was blocked by rabbit anti-Anabaena azollae serum.