I studied host resistance to infection in critically ill patients with nosocomial pneumonia due to gram-negative bacilli. The bactericidal activity of polymorphonuclear leukocytes (PMNs) against pathogenic bacteria was estimated by zymosan-induced and bacteria-induced chemiluminescence (CL). The function of B lymphocytes was examined by measurement of lymphocyte numbers, subpopulations, subsets, and plasma immunoglobulins. The zymosaninduced CL did not correlate significantly with the bacteria-induced CL. This finding indicates that zymosan cannot be used for the estimation of bactericidal activity of PMN against pathogenic bacteria in critically ill patients. Lymphocyte numbers were very low, and the parcentage of suppressor T lymhocytes was low, and consequently the ratio of helper to suppressor T lymphocytes was somewhat high. There was a significant negative correlation between the plasma level of IgG and the percentage of B lymphocytes (p<0.05). The bactericidal activity of PMNs and the function of B lymphocytes tended to be suppressed as the plasma level of IgG increased. From these results, I conclude that: 1. There is negative feedback between IgG and B lymphocytes. 2. Host resistance to infection is suppressed in critically ill patients, if IgG is elevated.