ObjectiveTo investigate the physicochemical properties and biological activity of self-prepared fusion protein inducible co-stimulator-Ig. MethodsAcid hydrolysis, edman degradation and peptide mass finger printing were used to determine the amino acid composition, N-terminal 15 amino acid sequences, and peptide mapping. In vivo mixed lymphocyte reaction assay was used for identification of its biological activity. ResultsThe result of amino acids composition analysis was consistent with the theoretical value of ICOS-Ig. N-terminal 15 amino acid sequences of the product were EINGSANYEMFIFHN, consistent with the theoretical value of ICOS-Ig. Peptide match assay identified six peptides of the product which could match the theoretic maps of ICOS-Ig. ICOS-Ig and CsA noticeably inhibited the proliferation of allo-reactive T cells in vivo. ConclusionThe prepared ICOS-Ig fusion protein has a correct structure and can inhibit the proliferation of allogeneic T cells in vivo, which lays a foundation for quality control of ICOS-Ig fusion protein.