A review. The author deals with the interaction between host and parasite in certain trypanosome and malarial infections, and, by limiting his consideration to those types of host resistance which exert a direct adverse effect upon the parasite, he shows how the different types of infections can be correlated with the production of antibodies on the part of the host. As the antibodies can produce their adverse effects either by killing the parasites after they are formed (para-siticidal effects) or by inhibiting the rate of reproduction, he gives a detailed account of the methods devised by him and his coworkers for differentiating the 2 effects of host resistance. The different types of infection are then discussed under the following heads review of the course of infection; application of methods previously noted for analysis of the effects of resistance; the immunological basis (antibodies) for the effects in as far as they are known. Two figures are given of malarial and trypanosome parasites and 6 graphs showing the course of infections. A bibliography of 53 titles is given.