Viability of Listeria monocytogenes suspended in either normal rabbit serum or listericidal factor is significantly reduced before cell lysis. In normal serum Listeria populations are initially reduced by clumping of viable bacteria, and the cells within these clumps are morphologically altered and killed by the listericidal factor in serum. This clumping is complement independent and does not occur after suspension in purified listericidal factor. In electron micrographs, densely stained amorphous material was seen surrounding clumped cells suspended in serum. Initial damage to Listeria suspended in either serum or listericidal factor appears to be confined to plasma and mesosomal membranes. The mesosomes are rapidly disorganized, and the plasma membrane becomes crenated and separates from the cell wall. Subsequently, the fibrillar organization of the nuclear region disrupts, and the cytoplasm increases in granularity. Studies utilizing Listeria protoplasts provided evidence in support of the plasma membrane as the site of listericidal action. Cell death may be related to loss in enzyme function after crenation of the plasma membrane and mesosomal dissolution.