The purpose of the investigation was to determine if coliform organisms isolated on membrane filters can be incubated at a humidity below the point of saturation. The investigation consisted of comparing the development of coliform organisms on membrane filters incubated at various controlled levels of humidity with the development of coliform organisms incubated in a saturated atmosphere. The effect of humidity on various types of differentiating media was considered simultaneously with the above investigation. Several suggested methods for the regulation of humidity were examined and evaluated and two new methods were developed. The final results indicate that it is possible to incubate coliform organisms on membrane filters at all levels of humidity above 35 percent saturation. There was no significant difference noted in the results obtained on the four differentiating mediums: Two Step Endo, Single Step Endo, Hajna-Damon Endo and Dehydrated Schedule Nutrient. Gooch tubing provides a more nearly watertight seal in the membrane filter procedure than either Parafilm or Plastic Disposable Petri dishes. The relative humidity of a standard incubator may be regulated to any desired level up to about 85 percent by exposing a definite water surface area per unit volume of incubator space. This procedure permits ordinary incubators to be used for membrane filter examinations. / Master of Science