We have designed a set of oligonucleotide primers to amplify the cDNA of mouse immunoglobulin heavy and light chain variable domains by the polymerase chain reaction. The primers incorporate restriction sites that allow the cDNA of the variable domains to be force-cloned for sequencing and expression. Here we have applied the technique to clone and sequence the variable domains of five hybridoma antibodies and to express a mouse-human chimeric antibody that binds to the human mammary carcinoma line MCF-7. The technique should also lead to the cloning of antigen-binding specificities directly from immunoglobulin genes.