the presence of antimicrobial substances on human skin was investigated. Staphyloccus aureus (10-4 colony-forming units) was applied on the forearm of 50 subjects and covered with a semiocclusive device for 24 hr. In 54% of the subjects the organisms persisted, and in 34% S. aureus was inhibited on the skin. Subjects with persistent S. aureus also had persistent Candida albicans, and vice persa. This correlation was not noted with Streptococcus pyogenes. Skin lipids from the two groups of subjects were extracted with acetone and assayed against S. aureus, S. pyogenes, and C. albicans. The percentage of S. aureus or C. albicans recovered was higher (79% and 55%, respectively) in subjects with persistent microrgamisms on their skin than in those without (47% and 28%, respectively). Subjects with persistent S. aureus and C. albicans had higher counts of normal flora (average, 9.2 times 10-3) than those on whose skin these organisms did not persist (average, 7.4 times 10-2). Coagulase-negative Staphylococcus and Micrococcus were found in higher proportions in subjects with persistent test organisms than in those without. Subjects with lower counts of their normal flora had a higher proportion of diphtheroids (34%) than the high-count group (12%).