We studied both in vivo and in vitro specific host defense mechanisms in patients suffering from chronic post-traumatic osteomyelitis (n = 26). The cell-mediated immunity in vivo was impaired as indicated by the reduced reactivity in the delayed type hypersensitivity skin test. The concanavalin A or phytohemagglutinin-induced T-cell proliferation in vitro was markedly decreased in comparison to healthy donors. In contrast, B-cell proliferation stimulated by Staphylococcus aureus Cowan I was not altered. While the absolute lymphocyte counts and the percentage of T, B, and O cells were within the normal range, nine out of the 26 patients showed a significantly diminished ratio of CD4+ and CD8+ T cells. Humoral immunity in the patients was less affected as assessed by the unchanged serum levels of immunoglobulins (Ig). However, the T-cell dependent polyclonal Ig synthesis after in vitro stimulation with pokeweed mitogen was suppressed. Our results provide evidence that cell-mediated immune functions are predominantly impaired in patients with post-traumatic osteomyelitis which may contribute to the persistence of the localized bone infection.