We have subcloned the in vitro-adapted murine B cell leukemia, BCL1.B1, to obtain a variant that expresses both IgM and IgG1. By fluorescence analysis, radioiodination, and immunoprecipitation of cell surface Ig, and by RIA of medium from limiting dilution cultures, we have shown that: (a) all the cells express and secrete both isotypes. The heavy chains of both IgG1 and IgM have the apparent molecular weights of membrane mu and gamma 1 chains; (b) both isotypes bear the same idiotype as determined by immunoprecipitation with antiidiotypic antibody, and both use the same VDJ rearrangement as shown by Southern blotting; and (c) the cells express the membrane and secreted forms of mRNA for both mu and gamma 1 but not gamma 2b or gamma 3. Taken together, the data suggest that all the cells are synthesizing, expressing on their surface, and secreting two isotypes that use the same VDJ rearrangement in the DNA and express the same serologically-defined idiotype. The molecular basis responsible for the production of the two isotypes in a single cell is the subject of the accompanying paper.