The V-gene segment of the immunoglobulin heavy chain of the immunoglobulin heavy chain of the mouse B-cell hybridoma VI41 secreting a VSV-neutralizing antibody was cloned into an expression vector encoding the mouse mu-chain of allotype a. Transgenic mice express the transgenic mu-chain of the allotype a; endogenous IgM of the allotype b and immunoglobulins of other subclasses were not detected in their serum. (J:44933)