Stable factors I, II, IV, and V of Listeria monocytogenes are not affected by prolonged exposure to 100 C or by high concentrations of alcohol. Stable factor III is variously affected by heat and alcohol in different strains but does not lose its serological identity. It is suggested that the variable behavior of factor III may be explained on the basis of steric hindrance on the part of adjacent molecules in the cell matrix. Spontaneously occurring, smooth, non-motile variants are the material of choice for antigenic analysis of stable factors.