AbstractThe influence of two different types ofEscherichia coliKl capsule preparation on the phagosytosis of anE. colilaboratory strain (LN 28) by polymorphnuclear leucocytes (PMNL) is described.Capsule material ofE. coli0 18 and 0 83 was prepared a) from the culture supernatant and b) from a bacterial pellet fraction. When capsule material from the pellet was added to opsonin, before mixing the laboratory strain with the PMNL, it decreased the rate of phagocytosis, compared with untreated opsonin. The preparation from the supernatant of capsule showed no alteration of phagocytosic rate or capacity.These results are explained by the different chemical compositions of the preparations.