1. [2(-14)C]Mevalonic acid injected into the echinoderm Asterias rubens (Class Asteroidea) was effectively incorporated into the non-saponifiable lipid. 2. The most extensively labelled compounds were squalene and the 4,4-dimethyl sterols with much lower incorporations into the 4α-monomethyl and 4-demethyl sterol fractions. 3. Labelled compounds identified were squalene, lanosterol, 4,4-dimethyl-5α-cholesta-8,24-dien-3β-ol and 4α-methyl-5α-cholest-7-en-3β-ol; these are all intermediates in sterol biosynthesis. 4. The major sterol in A. rubens, 5α-cholest-7-en-3β-ol, was also labelled showing that this echinoderm is capable of sterol biosynthesis de novo. 5. No evidence was obtained for the incorporation of [2(-14)C]mevalonic acid into the C28 and C29 components of the 4-demethyl sterols or 9β,19-cyclopropane sterols found in A. rubens and it is assumed that these sterols are of dietary origin. 6. Another starfish Henricia sanguinolenta also incorporated [2(-14)C]mevalonic acid into squalene and lanosterol. 7. Various isolated tissues of A. rubens were all capable of incorporation of [2(-14)C]mevalonic acid into the nonsaponifiable lipid. With the body-wall and stomach tissues radioactivity accumulated in squalene and the 4,4-dimethyl sterols, but with the gonads and pyloric caecae there was a more efficient incorporation of radioactivity into the 4-demethyl sterols, principally 5α-cholest-7-en-3β-ol.

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