Three phenolic constituents of Magnolia grandiflora L. were shown to possess significant antimicrobial activity using an agar well diffusion assay. Magnolol, honokiol, and 3.5'-diallyl-2'-hydroxy-4-methoxybiphenyl exhibited significant activity against Gram-positive and acid-fast bacteria and fungi. The minimum inhibitory concentrations were determined for each compound using a twofold serial dilution assay.