ABSTRACT. Two lines of immortal human fibroblasts were isolated following transfection of TIG-3 cells with plasmid DNA,PMT-lODfaA, that contained SV40early gene with a deletion in replication origin and ts mutation in coding sequence for T-antigen. These cells continued proliferation at 34°C, over 565 population doubling level (PDL) which is far over the limited division potential of untransformed normal TIG-3 of 70-80 PDL. When the culture temperature was shifted to 40°C after 70 PDL, they ceased proliferation immediately. One of these immortal clones, SV&8, lost its ts phenotype after retransformation with HtfT-antigen gene. These results indicated that the function of intact T-antigen is required for maintenance of immortal proliferation, at least in one of the SV40transformed immortal clones.

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