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Infection by the intestinal nematode parasite Nematospiroides dubius was studied in seven different inbred mouse strains. Although there was some minor variation in the susceptibility of the different strains to a primary infection there were marked differences in their ability to develop resistance to infection following repeated exposure to infective larvae. The strains of mice which developed the best resistance also expelled adult worms arising from the previous infections. The adult worms resulting from a primary infection were slowly eliminated in two inbred strains studied whereas no loss occurred from outbred LACA mice. Although males and females of two strains, C3H/HeJ and CBA/H were equally susceptible to a primary infection, the females developed better resistance than the male mice following two oral administrations of third stage larvae. Infected mice of every strain and both sexes contained high levels of IgG1 in the serum.

Virulent strains of Salmonella opsonized with normal mouse plasma are never phagocytosed as well as avirulent strains. The virulent strains of Salmonella phagocytosed after opsonization with normal mouse plasma are able to multiply within normal mouse peritoneal macrophages, whereas under similar experimental conditions the avirulent strains are killed. When virulent strains of Salmonella are opsonized with specific antiserum or plasma from BCG-infected mice, they are treated by normal mouse macrophages as if they were avirulent. Virulent bacteria opsonized with BCG plasma are phagocytosed and killed better by peritoneal macrophages from BCG-infected mice, than peritoneal macrophages from normal mice.

An investigation of the clearance of bacteria and colloids from the blood stream of mice has shown that both living and inert particles require serum factors (opsonins) in order that they may be phagocytosed by the macrophages of the reticuloendothelial system. It has been demonstrated that after the injection of a large dose of colloid there is a depletion of these serum opsonins which appears to account for the reduced rate of clearance of a second dose of colloid or living bacteria, since replacement of these factors leads to normal clearance. The significance of these results is discussed and it is suggested that in "blockaded" animals there is a depletion of serum opsonins rather than a saturation of phagocytic cells.