Résumé : The Simian virus 40‐transformed rabbit spleen cell TRSC‐1 synthesizes intracellular whole IgG molecules of the a1b4 allotype. Two hypoxanthine‐guanine phosphoryl transferase‐deficient mutants were derived from this line. One of these, TRSC‐1‐8, was used in somatic cell fusion experiments together with gangliocytes from a rabbit immunized against β‐galactosidase. Out of nineteen hybrid clones surviving in selective medium, only one, L17, was shown to produce free γ chains which express the a2 allotype of the donor rabbit rather than the a1 marker of the parents TRSC‐1‐8 line. The inability to restore IgG secretion in hybrids suggests that dominant regulatory controls are exerted by the TRSC‐1 genome on Ig production. This supports the notion that the TRSC‐1 line originated from a splenocyte that had not reached the final plasmocyte differentiation stage at the time of viral transformation. Copyright © 1981 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim