The percent lipid composition of pooled human sebum analyzed by thin-layer chromatography was: ceramides (13%), fatty acid (47%), cholesterol (7%), cholesterol esters (2%), squalene (11%), triglycerides (3%), and wax esters (17%). Total sebum lipids (2– 4 mg/ml), sonicated into bacterial culture medium, caused 4- to 5-fold log reduction in growth of gram-positive bacteria, Staphylococcus aureus, Streptococcus salivarius and the anaerobe Fusobacterium nucleatum, but was ineffective against most gram-negative bacteria. Fractionation of the sebum lipids showed that both saturated and unsaturated fatty acids contained the bulk of the antimicrobial activity. Lauric acid (C12:0) was the most active saturated fatty acid. The unsaturated fatty acid, palmitoleic acid (C16:1Δ6, cPA) was both the most predominant monoene and the most active antimicrobial fatty acid. Purified cPA (>99%) yielded typical minimal inhibitory concentration (MIC) values of 10–20 µg/ml against gram-positive bacteria. Organically synthesized cPA isomer gave MIC values comparable to the natural material. Both natural and synthetic cPA were found to be the most active sebum lipid fraction in blocking the adherence of a pathogenic strain of Candida albicans to porcine stratum corneum. Ethanol in combination with cPA exerts a synergistic bactericidal activity against gram-negative pathogenic bacteria, including Pseudomonas aeruginosa, Propionibacterium acnes, Escherichia coli, and several methacillin-resistant strains of S. aureus. Palmitoleic acid may be useful in topical formulations for treatment of secondary gram-positive bacterial infections, as a gram-positive bacteria antimicrobial in wound dressings, and as a natural gram-positive antimicrobial preservative in skin and hair care products.