The effects of a 3-day test of an occlusion dressing of the forearm on the resident flora of the skin, relative skin-moisture and skin-pH were studied in 26 male and female volunteers. The geometrical mean values of coagulase-negative Staphylococci and Corynebacterium species increased five and four logs and decreased 1 day post occlusion only two logs. Relative skin-moisture increased from 20% to 75% under occlusion and decreased to 5% 24h after removal. Skin-pH increased from 4.9 to 7.1 and decreased to pH 5.2. Investigations of the resident flora should be made, if the influence of external, not antimicrobial active agents, is to be examined, because changes in the substrate can cause secondary changes in the resident flora.