The idea that the lymphatic system, like the blood vessel system, may be affected by bacillus infection is quite reasonable. Thus we started basic investigations in determining the relationship between the lymphatic system and bacillus infection. To begin with, basic experiments were conducted to compare the bactericidal power of lymph with that of blood.As the first step, lymph was collected from ductus thoracicus of a rabbit; and several strains of Staphylococcus, Salmonella and Shigella, were chosen to test bectericidal properties of lymph against such bacilli, in comparison with that of serum. In our experiments, lymph proved, contrary to our expectation, equal to and even more powerful than blood in bactericidal properties against certain strain. Next, in order to determine the bactericidal substances, lymph and serum were inactivated by heating at 56°C for 30 min. Also, in order to investigate whether or not these bactericidal substances were the same as properdin system, RP was made from serum and from lymph, according to the method of Pillmer, with zymosan applied, to determine bactericidal effects. At inactivation of 56°C, 30 min. and in RP, bacterididal effects of serum and lymph showed marked reduction against Gram-negative bacteria. It is, therefore, presumed that properdin system plays an important role as bactericidal properties in lymph as well as in serum. On the other hand, in absorption experiments with the heated bacteria, the bactericidal effects of lymph and serum showed marked reduction, especially against the same bacteria, suggesting a factor like natural antibody.On comparing agglutination values and bactericidal effects of lymph with those of serum, after immunization by each strains of S. typhi Watson V and S. choleraesuis No.2, the increase in agglutination values was observed almost same as serum, but bactericidal power was inhibited specifically against immunized becteria, similarly to Neisser-Wechsberg's counter phenomenon in serum.