Inclusion of vitamin E (DL-alpha-tocopherol) in the culture medium for human diploid cells greatly prolongs their in vitro lifespan. The addition of 100 mug of DL-alpha-tocopherol per ml of medium has allowed us to culture WI-38 cells for more than 100 population doublings to date. (These cells normally have an in vitro lifespan of 50 +/- 10 population doublings.) Cells at the 100th population doubling have a normal diploid karyotype, appear to behave in all other respects like young WI-38 cells, and are still actively dividing. We interpret this result as support for the free radical theory of aging.