Antibody chains are encoded in three gene clusters containing genes for the variable and constant regions. V and C genes are separated in germ line and during differentiation a rearrangement takes place. But even after this rearrangement the V and C coding sequences are not contiguous. A final splicing must take place in committed cells between the transcription of a discontinuous V-and C-region DNA and the expression of a continuous mRNA coding for an antibody chain. Analysis by cell fusion indicates that the splicing is cis. When two antibody-producing cell lines are fused, the resulting hybrids express the two antibodies that characterize the parental lines. Permanent cell lines producing antibody of predefined specificity have now been derived in this way. Spleen cells from hyperimmunized donors are fused with myeloma cells and a proportion of the hybrids that are established synthesize and secrete antibodies directed against the immunogen. The heterogeneous cell population can be cloned and propagated. This is a potent way of producing monospecific antibodies to complex antigens such as cell membranes and transplantation antigens. Monoclonal xenogeneic antibodies to rat cell-surface membranes have proved very valuable for characterizing and separating rat lymphocyte subpopulations. In more recent experiments, monoclonal xenogeneic antibodies to mouse and human cell-surface antigens have also been produced which permit the characterization of the hitherto undescribed differentiation antigens.