A fraction of the ultrafilterable portion of chick embryo extract was isolated by alcohol extraction of a lyophilized powder of the ultrafiltrate followed by ion exchange removal of many of the inert components of the alcohol extract. This fraction contained 3 per cent of the ultrafilterable nitrogen but was capable of completely restoring the growth-promoting activity of dialyzed embryo extract, when tested with chick heart fibroblasts in roller tube cultures.

The low nitrogen content, shape of the ultraviolet absorption spectrum, and presence of few free amino acids, suggest that non-dialyzable compounds serve as the chief source of nutrition for this system.

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