Molecules of polysaccharide, lipopolysaccharide, and peptidoglycane from Pseudomonas aeruginosa were assayed as immunogens and immunomodifiers. The kinetics of humoral responses, phagocytic activity, splenic index variations, and immunomodulating ability of these molecules were studied. Their protective and immunomodifying effects were compared to those similarly produced by heat-killed bacteria suspensions. The influence of the nature and doses of these molecules as well as the intervals between their administration and the SRBC immunization were also studied. Results showed that higher IgM and IgG levels were produced by lower doses of lipopolysaccharide and peptidoglycane, respectively. Using these amphiphilic molecules at doses of 160 micrograms/mouse, a variable immunodepressive activity was observed. This dose-dependent effect which particularly altered the IgM synthesis, was also observed in phagocytic responses. The exopolysaccharide showed to be the most immunodepressive molecule, and this activity appeared to be dependent of dose and time of administration.