Joyner, A. E., Jr. (University of California, Davis), and R. L. Baldwin. Enzymatic studies of pure cultures of rumen microorganisms. J. Bacteriol. 92:1321-1330. 1966.-The activities of enzymes representing the major pathways of carbohydrate metabolism and anaerobic electron transport in cell-free extracts of whole rumen contents have been reported. The effects of diet upon the activities of several enzymes suggested that enzymatic measurements might prove useful for the study of rumen metabolism. In the present study, the distribution and characteristics of aldolase, succinate dehydrogenase, glutamate dehydrogenase, lactyl-coenzyme A dehydrase, lactate dehydrogenase, and other enzymes were measured in cell-free extracts of pure cultures of Ruminococcus flavefaciens, R. albus, Bacteroides succinogenes, B. ruminicola, B. amylophilus, Butyrivibrio fibrisolvens, Peptostreptococcus elsdenii, Streptococcus bovis, and Selenomonas ruminantium. Some enzymes were widely distributed (aldolase, glutamate dehydrogenase), whereas others were observed in one or two species (lactyl-coenzyme A dehydrase). The cofactor requirements and kinetic characteristics of enzymes varied considerably with species. Enzymes that vary with species might be employed as indices for estimating the activities of various groups of microorganisms in whole rumen contents.