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WHILE the importance of amino-acids1,2, salts3and vitamins4for growth of mammalian cells in culture is well established, the role of lipids remains uncertain. With the exception of free fatty acids5, no naturally occurring lipids such as glycerides, phospholipids or cholesterol have been shown to promote growth of mammalian cells in vitro.

WHEN disaggregated mouse embryo cells are first cultivated in vitro, the cells grow into a confluent layer of fibroblasts on the bottom of the Petri dish, and then begin to grow additional layers on top of the first. If the cells are transferred, subsequent subcultures will show less tendency to form multiple layers. This effect is accompanied by a diminution in growth-rate of the cells and may be described as a progressive decrease in their growth potential. This behaviour is to be contrasted with that of L cells, an established cell line derived from mouse fibroblasts, which show no diminution in their growth potential upon transfer, and retain their ability to grow out of the monolayer.