Leonard Hayflick, PhD, is a past president of the Gerontological Society of America and was a founding member of the council of the National Institute on Aging (NIA). The recipient of a number of research prizes and awards, including the 1991 Sandoz Prize for Gerontological Research, he has studied the aging process for more than fifty years. He is known for discovering that normal human cells divide for a limited number of times in vitro (refuting the contention by Alexis Carrel that normal body cells are immortal). This is known as the Hayflick limit. His discoveries overturned a 60-year old dogma that all cultured cells are immortal. Hayflick demonstrated that normal cells have a memory and can remember at what doubling level they have reached. He demonstrated that his normal human cell strains were free from contaminating viruses. His cell strain WI-38 soon replaced primary monkey kidney cells and became the substrate for the production of most of the world's human virus vaccines. Hayflick discovered that the etiological agent of primary atypical pneumonia (also called "walking pneumonia") was not a virus as previously believed. He was the first to cultivate the causative organism called a mycoplasma, the smallest free-living organism, which Hayflick isolated on a unique culture medium that bears his name. He named the organism Mycoplasma pneumoniae.