We investigated the role of cytokines in differences in histopathologic changes in the lung between bronchopneumonia caused by non-encapsulated Haemophilus influenzae strain 770235f 0 b 0 and systemic disease caused by type b H. influenzae strain 770235f 0 b + . Tumour necrosis factor-α (TNF-α), interleukin-(IL)-6 and IL-1β levels in bronchoalveolar lavage fluid (BALF) samples of mice infected with strain 770235f 0 b 0 were higher than in those infected with strain 770235f 0 b + until 24 h post-infection. Serum IL-6 rapidly increased in strain 770235f 0 b 0 infection after 72 h post-infection. Serum TNF-α level in strain 770235f 0 b 0 infection appeared earlier than in strain 770235f 0 b + infection. IL-1β production in strain 770235f 0 b + infection was later than in strain 770235f 0 b 0 infection. Moreover, a biphasic concentration pattern of TNF-α and IL-6 was noted in BALF of mice with strain 770235f 0 b 0 infection.