The recently established culture medium, GIT, is applicable to many kinds of cells including mouse and human myeloma cells, and most adhesive cell lines. We applied this GIT medium to mouse B cell hybridoma production. When the medium was used to propagate myeloma cells before cell fusion and also for HAT selective medium, the fusion efficiency was more than twice as high as when the regular medium (RPMI-1640 supplemented with FBS) was used. Constantly more than 80% wells were hybridoma positive irrespective of the antigens used. To determine the optimal cell concentration at the hybridoma selection, a graded number of myeloma and spleen cells was distributed to each well ; the best result was obtained when 3 X 104 myeloma and 3 X 105 spleen cells were distributed to each well. In addition, the GIT medium shows very little lot-to-lot variation. These results indicate that fusion efficiency in mouse B cell hybridoma production was greatly improved by using GIT medium. hybridoma production ; GIT medium ; monoclonal antibody Since Kohler and Milstein (1975) introduced a new technology to obtain a monoclonal antibody directed against a single epitope on an antigen, monoclonal antibodies have been used extensively. The production of monoclonal antibodies to various antigens by the fusion of antigen-sensitized spleen cells with myeloma cells has been expanded to every biological field. Though basic methods have been well established, there is still a need for the more effective, stable fusion efficiency. Good and constant fusion efficiency makes it possible to effectively establish desired hybridoma clones and contributes markedly to the progress of