Crude cell wall (CCW), enzyme purified wall (PF), peptidoglycans (PEP), and teichoic acid fractions from two strains of Listeria monocytogenes (EGD and a human isolate) were analyzed by electron microscopy, chemical analyses, and various immunological assays. Electron microscopy and chemical analyses revealed that both strains were quite similar. The CCW, PF, and PEP fractions were mitogenic for mouse lymphocytes, could activate macrophages in vivo but not in vitro, and stimulated nonspecific immunity in vivo to Candida albicans. The PF and PEP fractions but not the CCW induced migration inhibition factor production. The CCW and PEP fractions were tested and found to elevate natural killer activity in vivo. Teichoic acid fractions had no activity in the above assays. Since most of the immunological activities were retained in the PEP fraction, we examined this fraction to determine the kinetics of the mitogenic response and the nature of the responding cell population. The peak mitogenic response occurred on day 2 over a 6-day period. This response was not enriched in T-cell-enriched populations but followed the pattern of LPS in B-cell-enriched populations. * Corresponding author. 170 sterile wire screen into 5 ml of cold tissue culture medium.