The synthesis of intracellular and secretory proteins by rat hepatoma (MHC) and mouse teratocarcinoma (PCC4AZAl) cells and MHC X PCC4AZAl somatic cell hybrids was examined with two-dimensional (O'Farrell) electrophoresis. The gels of the PCC4AZAl and hybrid cells were nearly identical and were quite different from those of the MHC cells. The teratocarcinoma phenotype was, therefore, dominant in the teratocarcinoma X hepatoma somatic cell hybrids.