The growth requirement or growth-promoting effect of biotin-vitamers on bacteria and yeasts was investigated. Biotin, dethiobiotin and biocytin (N-ε-biotinyl-L-lysine) were shown to be required for growth in a number of bacteria and yeasts. The biological activity of dethiobiotin was relatively higher than that of biotin, but was negative for lactic acid bacteria. Biocytin had high activity for Bacillus subtilis (natto), Debaryomyces japonicus and Hansenula capsulata. The biotin activity of 7-keto-8-aminopelargonic and 7, 8-diaminopelargonic acids was low or negligible for bacteria but relatively high for yeasts such as the genera of Endomyces, Endomycopsis and Saccharomyces. Pimelic, pelargonic and pelargonylhydroxamic acids had no growth requirement for or growth-promoting effect on any of the bacteria or yeasts tested.