The invention provides a pharmaceutical composition which comprises a pharmaceutical carrier and at least one compound selected from the group consisting of cobaltocene-octamethyl, stigmastan- 3,5 -diene, galoxolide, benzyl salicylate, eucalyptol, and .alpha.-pinene. Also provided is a method of preparing a composition having antimicrobial activity comprising extracting a plant material in an organic solvent, contacting the extracted material to a chromatographic column, and eluting from the chromatographic column with a mobile polar phase to obtain a composition. The plant material is from Mammea Americana, Marchantaceae polymorpha, or Callistemon citrinus. Also provided is a method of inhibiting the growth of a mycobacterium, comprising administering a composition comprising a carrier and at least one compound selected from among cobaltocene-octamethyl, stigmastan, 3,5 -diene, friedelin, galoxolide, benzyl salicylate, eucalyptol, and .alpha.-pinene.