Abstract
Fusions were performed between myeloma cell lines, of mouse and rat origin, and mouse or rat spleen cells. Two statistical methods have been used to measure the proportion of hybrids expressing a spleen cellderived immunoglobulin phenotype, one of them applicable to cells growing under nonlimiting dilution conditions. The results indicate that there is strong preferential selection for hybrid cell growth with an immunoglobulin-secreting phenotype. The degree of preferential selection is dependent upon the myeloma cell line used and is most marked in the case of the rat myeloma lines. Surviving hybrids seem to originate from fusions of myeloma and spleen B (but not T) cells, but immunoglobulin production is lost more readily in certain combinations.