1. The hydrogenation of all the methylene-interrupted cis, cis-octadecadienoic acids was examined using pure cultures of six rumen bacteria able to hydrogenate linoleic acid to stearic acid or its immediate precursor, trans-1 1-octadecenoic acid, after first conjugating the linoleic acid to cis,truns-9, 1 1-octadecadienoic acid.

2. Only the Δ 14-cis, 17-cis-isomer was not hydrogenated by at least one of the bacteria and no evidence was found that conjugation was necessary before hydrogenation except for the Δ 2-cis,5-cis- and Δ 9-cis, 12-cis-isomers. Several isomers were hydrogenated to an extent close to that achieved with linoleic acid (Δ 9-cis, 124s).

3. Those bacteria only able to hydrogenate linoleic to trans-1 1-octadecenoic acid gave only octadecenoic acid products and those bacteria able to hydrogenate linoleic acid to stearic gave variable yields of octadecenoic acids and stearic acid except with the isomers Δ 12-cis, 15-cis and Δ 13-cis, Idcis when only octadecenoic acids were detected.

4. At the substrate levels used (20 μg/ml), both inhibition and stimulation of growth were found but no common pattern emerged, nor was the growth consistently related to the extent of hydrogenation.