Trauma results in concomitant immunosuppression and elevated monocyte (M phi) inflammatory cytokine levels. The augmenting or ameliorating effect of IL-10 in septic complications after trauma is controversial. Here, IL-10 levels of trauma patients' and normals' PBMC, isolated M phi, and isolated T cells were assessed and correlated to their PBMC mitogen responses, their T-cell proliferation in an APC independent system, and their M phi production of elevated TNF-alpha levels. Trauma patients with depressed PBMC responses to PHA stimulation also had significantly decreased IL-10 levels in their stimulated PBMC supernates (P = 0.0022) and their MDP-stimulated isolated M phi population (P = 0.0004). However, patients with depressed PHA responses could have either normal or depressed T-cell proliferation in an anti-CD3-, anti-CD4-stimulated system. If APC-independent T-cell proliferation was depressed, induced IL-10 levels were suppressed (P = 0.007). However, if APC-independent T-cell proliferation was normal or elevated, IL-10 levels could be normal or elevated (P = 0.018). Decreased IL-10 levels correlated with depressed mitogen responses and depressed T-cell proliferation. IL-10, therefore, could not be inducing trauma patients' immunosuppression. Patients with elevated M phi TNF-alpha levels had depressed M phi IL-10 levels.