Hybridomas producing monoclonal antibody (MAb) of predefined specificity were isolated from a Chinese hamster that had received injections of a Chinese hamster-human somatic cell hybrid. A standard method for the production of murine hybridomas was used to produce Chinese spleen lymphocyte X murine plasmacytoma hybridomas, and they were screened for complement-mediated cytotoxicity against a panel of Chinese hamster-human somatic cell hybrids and agglutination of human erythrocytes. Two hybridomas were established in tissue culture following limiting dilution cloning, and their reactivity with a panel of Chinese hamster-human somatic cell hybrids indicates that they are specific for the previously identified human cell-surface antigen a1. The Chinese hamster appears to respond preferentially to human antigens of Chinese hamster-human somatic cell hybrids, and it will serve as a useful tool for the production of MAb specific for human cell-surface antigens expressed in the many well-characterized Chinese hamster-human somatic cell hybrids available.