Meningococcal sepsis results partly from overproduction of host cytokines after macrophages interact with endotoxin. To obtain less toxic and highly immunomodulatory meningococcal endotoxins for prophylactic purposes, we investigated the relationship between endotoxicity and immunomodulatory activity of several endotoxin preparations from Neisseria meningitidis group B. Using the D-galactosamine-sensitized mouse model to determine endotoxin lethality, we found that the toxicity of purified lipooligosaccharide (LOS) from M986, a group B disease strain, was three to four times higher than those of purified LOSs from the noncapsulated strains M986-NCV-1 and OP-, the truncated-LOS mutant. The LOSs of outer membrane vesicles (OMVs) and detergent-treated OMVs (D-OMVs) from the three strains were 2 to 3 and over 300 times less toxic than the purified LOSs, respectively. Intraperitoneal administration of these preparations induced production of tumor necrosis factor alpha (TNF-alpha) and interleukin 6 (IL-6) in serum 2 h after injections. However, repeated doses of low- and high-toxicity preparations induced lower amounts of TNF-alpha and IL-6, i.e., LOS tolerance. Injection of mice with low doses of LOS was as effective as injection with high doses in inducing tolerance. Peritoneal macrophages from tolerant mice pretreated with either high- or low-toxicity LOS preparations produced only a fraction of the amounts of TNF-alpha and IL-6 produced by control groups in response to LOS ex vivo. Despite tolerance to LOS induced by pretreatment with reduced-toxicity preparations, killing of N. meningitidis M986 by macrophages from these animals was enhanced. Protection was achieved when mice treated with LOS, and especially that of D-OMVs, were challenged with live N. meningitidis. The least toxic LOS, that in D-OMVs, was most effective in inducing hyporesponsiveness to endotoxin in mice but protected them against challenge with N. meningitidis. No inevitable link between toxicity and host immune modulation and responses was shown. Our results show that LOS is responsible for both toxicity and immunomodulation. When LOS is tightly associated with outer membrane proteins in D-OMV, it reduces toxicity but enhances beneficial effects compared to results with its purified form. Thus, systematic and critical evaluation of D-OMVs as adjuvants or as portions of group B meningococcal vaccines may help improve survival and outcome in meningococcal sepsis.