Hayflick is known for his research in cell biology, virus vaccine development, and mycoplasmology. In 1962 he discovered that, contrary to the prevailing belief at the time, cultured normal human and animal cells have a limited capacity for replication. This discovery, known as the Hayflick limit, overturned a long-held belief bolstered by Alexis Carrel's work in the early 20th century that claimed that normal cells would proliferate continuously in culture. Hayflick found that only cancer cells are immortal and that normal cells have a mechanism for remembering what replication level they are at. He interpreted his finding that normal cells are mortal, to be an indication of aging at the cellular level. Hayflick demonstrated for the first time that mortal (normal) and immortal (malignant) mammalian cells existed.