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(Ross, 1950(Ross, , 1952 but subsequently Hutner, Bach, and Ross (1956) introduced the z strain. Using this strain and an improved medium, much more growth is obtained, the incubation period is shorter, and the assay is more sensitive.The Euglena method has several advantages for assaying B12 in serum. It has a high specificity and is very sensitive. Because whole serum is assayed, preliminary extraction is unnecessary and direct measurements can be made of both bound and free B12. However, growth conditions need careful control and the results often vary widely in different laboratories (Table I) In the assay the serum B12 concentration is usually determined by comparing the growth of the organism in serum with its growth in aqueous

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