3T3 cells come from a cell line established in 1962 by two scientists then at the Department of Pathology in the New York University School of Medicine, George Todaro and Howard Green. The 3T3 cell line has become the standard fibroblast cell line. Todaro and Green originally obtained their 3T3 cells from Swiss mouse embryo tissue.
The '3T3' designation refers to the abbreviation of "3-day transfer, inoculum 3 x 10<sup>5</sup> cells." This cell line was originally established from the primary mouse embryonic fibroblast cells that were cultured by the designated protocol, so-called '3T3 protocol'. The primary mouse embryonic fibroblast cells were transferred (the "T") every 3 days (the first "3"), and inoculated at the rigid density of 3 x 10<sup>5</sup> cells per 20-cm² dish (the second "3") continuously. The spontaneously immortalized cells with stable growth rate were established after 20-30 generations in culture, and then named '3T3' cells.
3T3 cells are often used in the cultivation of keratinocytes, with the 3T3 cells secreting growth factors favourable to these kinds of cells.