SummaryThe underlying unity between the normal growth requirements of micro‐organisms and those of animals is pointed out, and the wide range of organisms requiring unidentified members of the vitamin B complex is given.The preparation and chemical properties of a number of these unknown factors is described. The biological activity of many of these compounds has been assessed by their growth‐promoting properties forLactobacillus caseiE andStreptococcus lactisR. On this basis three compounds known as the norite eluate factors (orL. caseifactors) have been prepared from liver, yeast and fermentation residues with similar chemical properties but differing activity for these micro‐organisms. A synthetic material of undisclosed structure has been described and is claimed to be identical with the compound isolated from liver. From this work and that concerning folic acid, a factor obtained from spinach, it appears that there are a number of naturally occurring substances which bear a close chemical resemblance, but slight changes in structure cause considerable changes in biological activity.A factor (vitamin Bc) has been obtained from liver which is active forL. caseiE andS. lactisR and for normal growth, feathering and haematopoiesis in the chick, but its identity with any of the above factors is not proved. A more complex form of this substance has been obtained which is active for the chick but not for the micro‐organisms. A factor 2500 times more active forS. lactisR than forL. caseiE has been obtained from an unstated source.A factor (vitamin M) which cures nutritional cytopenia in monkeys has been prepared from liver and yeast, and this material can be converted into anS. lactisR‐stimulating factor. Vitamin Bc, vitamin M,L. caseifactor and folic acid are probably closely related, and some interrelationships in biological activity have been demonstrated.The total growth requirements ofL. caseiE and certain strains ofCorynebacterium diphtheriaefor unidentified factors have been studied by other workers, as distinct from the attempted isolation of any one of them, and the existence of at least five factors required by these micro‐organisms has been described. Their concentration from liver and outstanding chemical properties have been detailed. Similarities between theC. diphtheriaefactor, strepogenin–a factor required by certain strains of haemolytic streptococci–and a factor required by certain strains ofS. lactisare apparent. It has been shown that different factors are required byL. caseiE for the initiation of growth and for the production of lactic acid, and the second of these materials has been shown to consist of at least three factors, one of which is related to folic acid and the synthetic material of the ‘norite eluate’ factors.Apparent discrepancies in growth requirements of the micro‐organisms probably arise from unrecognized impurities in basal media, the enforced use of concentrates of active factors or the employment of cultures of differing age, resulting in differing growth requirements.Further factors are described, including a fat‐soluble factor or factors stimulatory forL. caseiE, and factors distinct from those mentioned above, required by the chick and guinea‐pig.Finally, the relationships between the unidentified factors and known compounds, especially purines, pyrimidines and pterins, in the nutrition of micro‐organisms are discussed. A structural resemblance between xanthopterin and folic acid has been indicated, and it has been shown possible to replace folic acid and vitamin Bccompletely for 5.lactisR and partially forL. caseiE by thymine, together with one of several purines.I am greatly indebted to Dr F. C. Happold and Mrs D. E. Dolby for much helpful criticism during the