The human HBL-100 cell line harbours SV40 DNA integrated in tandem at a unique site. The SV40 T-antigen expressed in these cells is defective in a function essential to the replication of the viral genome. The integrated SV40 sequences were molecularly cloned in a bacteriophage, and a subclone (plasmid pSVHBI) containing a complete SV40 DNA was isolated. As compared to SV40 wild-type strain 776, sequence analysis of pSVHBI early region revealed the presence of several DNA alterations. Among these, a point mutation at position 3199, predicting a change at amino-acid 540 of arginine to isoleucine, was shown by marker rescue to be responsible for the deficiency of T-antigen. This novel mutation further delimits one of the T-antigen domains involved in SV40 DNA replication. Transfection experiments demonstrated that the transforming activity of the SV40 genome from HBL-100 cells is still preserved. Moreover, several transformed human cell clones thus obtained could be permanently established in culture.