Antibodies against CD25 would be novel tools for the diagnosis and treatment of adult T cell leukemia lymphoma (ATLL) and many other immune disorders. In our previous work, we successfully produced the single-chain fragment of a variable antibody against CD25, the Dmab(scFv) antibody, using Pichia pastoris. Here, we describe a novel form of an antibody against CD25, the Dmab(scFv)-Fc antibody, also produced by P. pastoris. To construct the Dmab(scFv)-Fc antibody, the Dmab(scFv) antibody was genetically fused to the Fc fragment of a human IgG1 antibody. A fusion gene encoding Dmab(scFv)-Fc antibody was cloned into the pPIC9K plasmid and expressed at high levels, 60-70 mg/l, by P. pastoris under optimized conditions. The Dmab(scFv)-Fc antibody was similar to the Dmab(scFv) antibody in its binding specificity but different in its molecular form and Fc-mediated effector functions. The Dmab(scFv)-Fc antibody and the Dmab(scFv) antibody both bound to CD25-positive MJ cells but not to CD25-negative K562 cells. The Dmab(scFv)-Fc antibody existed as a dimer whereas the Dmab(scFv) antibody was a monomer because it lacks the Fc fragment. The Dmab(scFv)-Fc antibody enhanced the antibody-dependent cellular cytotoxicity of CD25-positive cancer cells, whereas the Dmab(scFv) antibody was inactive in the antibody-dependent cellular cytotoxicity assays. In addition, compared to the Dmab(scFv) antibody, the Dmab(scFv)-Fc antibody showed stronger immunosuppressive activity in the Con A-stimulated lymphocyte proliferation system and in the mixed lymphocyte reaction system. These results demonstrate that the Dmab(scFv)-Fc antibody produced in P. pastoris is functional, and therefore it might be developed as a novel diagnostic and therapeutic tool for ATLL and other immune disorders.