The reliability of a simple method of formaldehyde disinfection control, based on the colour intensity of End's agar or sulphite agar, was tested in a sealed fume chamber 1.2 cubic metres in size at a constant air temperature (21 degrees C) and at a 40-per-cent or 95-per-cent humidity. The effect of formaldehyde concentration and air humidity was examined, as exerted on bactericidal effectiveness and on the colour intensity of the mentioned media. Air humidity proved to be highly important: together with formaldehyde concentration and exposure time, air humidity is the decisive factor underlying the final effect of disinfection if due temperature is maintained. The intensity of the colouring of End's agar or sulphite agar was found to depend mainly on concentration and slightly on air humidity. Hence it is recommended that this simple control should be used only for the testing of a good sealing of the disinfected space underlying the effect of the active concentration of formaldehyde for the necessary exposure time. End's agar, produced by the Imuna National Corporation, Sarisské Michal'any, gave better results. It is considered necessary for an estimation of formaldehyde disinfection effectiveness to record, at the same time, the humidity and temperature of the air in the disinfected space. In our trials at a 95-per-cent humidity level, E. coli was totally devitalized on carriers disinfected with formaldehyde developed from 25 ml formaline and St. aureus with formaldehyde developed from more than 50 ml formaline per 1 cubic metre of space. When the air humidity level was 40% the total devitalization of the mentioned bacterial strains was not achieved even with formaldehyde concentration developed from 75 ml formaline per 1 cubic metre of space.