<jats:p> Two new antimetabolites of biotin were isolated from culture filtrates of <jats:italic>Streptomyces lydicus</jats:italic> : β-methyldethiobiotin and β-methylbiotin. <jats:sup>14</jats:sup> C-biotin or <jats:sup>14</jats:sup> C-pimelic acid was not incorporated into either of these antimetabolites by the growing culture. Neither of the compounds could substitute for the biotin requirement in <jats:italic>Saccharomyces cerevisiae</jats:italic> . Both compounds had a strong and rather specific antimicrobial effect against mycobacteria. Their antimicrobial activities were reversed by biotin. Both compounds had an affinity for avidin.