Continuous cultures of fused cells secreting antibody of predefined specificity.

Distinct plaques, each of which is due to the release of hemolysin by a single antibody-forming cell, are revealed by complement after incubation, in an agar layer, of a mixture of sheep red cells and lymphoid cells from a rabbit immunized with sheep red cells.

When two clonal lines of mouse fibroblasts, each containing a drug-resistant marker, are grown together for 4 days, hybrid cells can be detected by selective conditions. These hybrid cells are presumed to be the result of mating. By the same method evidence can be obtained which suggests that mating may be followed by segregation.