Three different methods were used for the monitoring of airborne microorganisms: (1). Cultivation of microbes trapped in a single-stage biological impactor directly on a solid agar nutrient medium (meat-pepton agar, Sabouraud's agar, blood agar) in Petri dishes. The repeated yearly course of concentrations of cultivable organisms, or colony-forming units (CFU), was obtained by long-run measurements. (2) Aeresol was trapped by impact on membrane filters, and the microorganisms were cultivated by placing the filters on the agar media as above. (3) Direct microorganism counting in a fluorescence microscope; air was sampled in a four-stage impactor where the aerosol was trapped on microscope slides, and the microorganisms were subsequently stained with fluorescent dyes (fluorescein diacctate, 4;6-diamidino-2-phenylindole and, particular, ethidium bromide).